Glow discharge heating apparatus

ABSTRACT

Typically, a bottom of a cup-shaped electrode opposes to that of a similar electrode through a predetermined gap and in a mixture of helium and hydrogen. A step-up transformer connected across an AC source through a resistor applies a voltage in excess of a discharge breakdown voltage for the gap across the electrodes to cause a pilot glow discharge between them before the source voltage reaches a glow hold minimum voltage for the electrodes. A rectified voltage resulting from a transformer connected across the resistor turns a bidirectional triode thyristor on to apply the source voltage across the electrodes through the conducting thyristor. This smoothly transits the pilot glow discharge to a glow discharge between the electrodes. The latter discharge heats a liquid forcedly flowing along inner surfaces of the electrodes. Also an auxiliary electrode can be operatively coupled to the electrodes to cause similarly a pilot glow discharge between it and either one of the electrodes. For a DC source a tubular anode surrounds a middle portion of a tubular cathode though which a liquid flows.

This is a divisional, of application Ser. No. 006,780, filed Jan. 26,1979.

BACKGROUND OF THE INVENTION

This invention relates to a glow discharge heating apparatus for heatinga liquid through the utilization of a glow discharge established betweena pair of electrodes involved.

Japanese laid-open patent application No. 6252/1976 describes and claimsa glow discharge heating apparatus for heating a liquid by utilizing aphenomenon that a glow discharge occurring between a pair of cathode andanode electrodes heats the cathode electrode to an elevated temperature.The glow discharge heating apparatus disclosed in the cited patentapplication comprises a hollow cylindrical enclosure, a tubular cathodeelectrode coaxially entended and sealed through the enclosure, andhaving both ends open, a hollow cylindrical anode electrode disposed inthe enclosure to surround the cathode electrode substantially throughourthe length thereof to form an annular discharge gap therebetween, asource of DC voltage connected across the cathode and anode electrodesto cause a glow discharge therebetween. The cathode electrode is heatedwith the glow discharge to directly heat a liquid flowing therethrough.

Heating apparatus of this type referred to have instantaneously heatedthe liquid with the simple construction and still with the highefficiency. However, where high currents are required to establish theglow discharge between the electrodes, it has been difficult to sustainthe stabilized glow discharge therebetween. There have been a fear thatthe glow discharge will transit to an arc discharge as the case may be.Also the electrodes have been heated to be axially expanded. This mightresult in a fear that the apparatus is broken.

Further it has been difficult to reliably control the glow dischargebecause of the absence of a control circuit for starting andextinguishing the glow discharge.

Accordingly it is an object of the present invention to eliminate thedisadvantages of the prior art practice as above described by theprovision of a new and improved glow discharge heating apparatus capableof always sustaining a stabilized glow discharge.

It is another object of the present invention to provide a new andimproved glow discharge heating apparatus including means for absorbingthermal strains developed in electrodes thereby to provide aconstruction difficult to be broken.

It is still another object of the present invention provide a new andimproved glow discharge heating apparatus including a control circuitfor easily controlling a glow discharge occurring across a pair ofelectrodes involbed.

SUMMARY OF THE INVENTION

The present invention provides a glow discharge heating apparatuscomprising at least one pair of discharge electrodes disposed inopposite relationship to form a predetermined gap therebetween, a sourceof voltage connected across the discharge electrodes to cause a glowdischarge therebetween, the glow discharge supplying electrical energyto at least one of the electrodes to heat the latter to an elevatedtemperature, and means for passing a liquid through the heated electrodeto heat the liquid wherein each of respective portions of the electrodesopposing to each other is less in surface area than a portion of eachelectrode having the glow discharge caused thereon to impart a positiveresistance to the current-to-voltage characteristic of the glowdischarge.

In a preferred embodiment of the present invention the source of voltagemay comprise a source of DC voltage and a hollow anode electrodesurrounds the middle portion of a hollow cathode electrode to form thepredetermined discharge gap therebetween, the cathode electrode forminga flow path for the heated liquid.

In another preferred embodiment of the present invention the source ofvoltage may comprise a source of AC voltage and the pair of electrodesare in the form of hollow cylinders having one end closed andsubstantially identical in shape to each other, the closed ends of thecylindrical electrodes abutting against each other to form thepredetermined gap therebetween while flow confining means is disposedwithin each electrode to flow the liquid in contact with and along theinternal surface thereof.

In order to ensure that the glow discharge is prevented form transitingto an arc discharge, the glow discharge heating apparatus mayadvantageously include an auxiliary source of voltage for applyingacross the electrodes a high voltage in excess of a discharge breakdownvoltage across the electrodes upon a discharge voltage across theelectrodes approaching a glow discharge-hold minimum voltage, to cause apilot glow discharge therebetween to induce the principal glowdischarge.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more readily apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a longitudinal sectional view of a glow discharge heatingapparatus constructed in accordance with the principles of the priorart;

FIG. 2A is a schematic sectional view of a pair of opposite electrodeuseful in explaining the glow discharge;

FIG. 2B is a graph illustrating a spatial voltage profile exhibited bythe arrangement shown in FIG. 2A;

FIG. 3 is a fragmental schematic plan view illustrating how a quantityof input heat to a cathode electrode during a glow discharge ismeasured;

FIG. 4 is a graph illustrating the results of the measurement shown inFIG. 3 with the results of a corresponding theoretical calculation;

FIG. 5 is a graph illustrating the relationship between a glow dischargevoltage and a gap length through which a glow discharge is caused;

FIG. 6 is a graph illustrating the relationship between a voltage and acurrent for the glow discharge;

FIG. 7 is a perspective view of a modeled ion flux useful in explaininga quantity of input heat to a cathode electrode resulting from a glowdischarge;

FIG. 8 is a graph illustrating the current-to-voltage characteristics ofthe glow discharge;

FIGS. 9A and 9B are fragmental schematic plan views of a pair ofopposite electrodes useful on explaining the principles of the presentinventions;

FIG. 10A, 10B and 10C are views similar to FIG. 9A or 9B butillustrating typically electrode configurations embodying the principlesof the present invention;

FIG. 11 is a longitudinal sectional view of one embodiment according tothe glow discharge heating apparatus of the present invention;

FIG. 12 is a current-to-voltage characteristic curve for a glowdischarge caused by the arrangement shown in FIG. 12;

FIGS. 13 and 14 are graphs useful in explaining the principles of thepresent invention;

FIG. 15 is a longitudinal sectional view of a modification of thearrangement shown in FIG. 11;

FIGS. 16 and 17 are graphs illustrating the characteristics of thearrangement shown in FIG. 15;

FIG. 18 is a longitudinal sectional view of another modification of thepresent invention;

FIG. 19 is a graph illustrating the characteristic of the arrangementshown in FIG. 18;

FIG. 20 shows a modification of the arrangement shown in FIG. 18 whereinFIG. 20A is a cross sectional view and FIGS. 20B and 20C are sideelevational views of the lefthand and righthand sides respectively;

FIG. 21 is a view similar to FIG. 18 but illustrating still anothermodification of the present invention;

FIG. 22 is a view similar to FIG. 18 but illustrating a modification ofthe arrangement shown in FIG. 21;

FIG. 23 is a view similar to FIG. 18 but illustrating anothermodification of the arrangement shown in FIG. 21;

FIG. 24 is a view similar to FIG. 18 but illustrating still anothermodification of the arrangement shown in FIG. 21;

FIG. 25 is a graph illustrating a leakage current calculated with thearrangement shown in FIG. 24;

FIG. 26 is a graphical representation of voltage and current waveformsdeveloped in the arrangement of FIG. 24 filled with a mixture of heliumand hydrogen;

FIG. 27 is graph illustrating the current-to-voltage characteristics ofglow discharges occurring in the arrangement of FIG. 24 filled withmixtures of helium and hydrogen having different proportions thereof;

FIG. 28 is a graph illustrating the theoretical relationship between aglow hold minimum voltage and quantity of input heat to an associatedelectrode resulting from the glow discharge;

FIG. 29 is a graph illustrating the relationship between an overlappingarea for both electrodes and a pressure of a filling gas;

FIG. 30, 31 and 32 is graphs illustrating how the glow hold minimumvoltage is changed with a proportion of mixed gases and a dischargegap-length;

FIG. 33 is a graph illustrating the relationship between the glow holdminimum voltage and a peak discharge current;

FIG. 34 is a longitudinal sectional view of a different modification ofthe present invention including an auxiliary electrode;

FIGS. 35, 36 and 37 are fragmental perspective views of differentmodifications of one of the electrodes shown in FIG. 34;

FIG. 38 is a longitudinal sectional view of modification of FIG. 34along with an associated electric circuit;

FIG. 39 is a longitudinal sectional view of another modification of thearrangement shown in FIG. 34;

FIG. 40 is a view similar to FIG. 38 but illustrating still anothermodification of the arrangement shown in FIG. 34;

FIG. 41 is view similar to FIG. 39 but illustrating a differentmodification of the arrangement shown in FIG. 34;

FIG. 42 is a view similar to FIG. 39 but illustrating a modification ofthe arrangement shown in FIG. 41;

FIG. 43 is a view similar to FIG. 39 but illustrating a modification ofthe arrangement shown in FIG. 40;

FIG. 44 is a view similar to FIG. 39 but illustrating anothermodification of the arrangement shown in FIG. 34;

FIG. 45 is a view similar to FIG. 39 but illustrating a modification ofthe arrangement shown in FIG. 44;

FIG. 46 is a diagram of the fundamental used with control circuit thepresent invention;

FIG. 47 is a graph illustrating a voltage and a current waveformdeveloped in the arrangement shown in FIG. 46;

FIG. 48 is a diagram of a control circuit constructed in accordance withthe principles of the present invention for driving the glow dischargeheating apparatus thereof;

FIG. 49 is a graph illustrating a voltage and a current waveformdeveloped in the arrangement shown in FIG. 48;

FIG. 50 is a diagram similar to FIG. 48 but illustrating a modificationof the arrangement shown in FIG. 48;

FIG. 51 is a graph similar to FIG. 49 but illustrating the arrangementshown in FIG. 50;

FIG. 52 is a diagram of another control circuit constructed inaccordance with the principles of the present invention and suitable foruse with an electrode structure including an auxiliary electrode;

FIG. 53 is a circuit diagram similar to FIG. 52 but illustrating amodification of the arrangement shown in FIG. 52;

FIG. 54 is a graph illustrating voltage waveforms developed at variouspoints in the arrangement shown in FIG. 52;

FIGS. 55 through 58 are circuit diagrams similar to FIG. 52 butillustrating different modifications of the arrangement shown in FIG.52;

FIG. 59 is a diagram of still another control circuit constructed inaccordance with the principles of the present invention;

FIG. 60 is a graph illustrating voltage waveforms developed in thearrangement shown in FIG. 59;

FIG. 61 is a graphical representation of a Laue plot;

FIG. 62 is a circuit diagram similar to FIG. 59 but illustrating amodification of the arrangement shown in FIG. 59;

FIG. 63 is a graph similar to FIG. 60 but illustrating the arrangementshown in FIG. 62;

FIG. 64 is a sectional view of an embodiment according to thethree-phase glow discharge heating apparatus of the present inventionand a diagram of a control circuit therefor;

FIG. 65 is a graph illustrating various waveforms developed in thearrangement shown in FIG. 64;

FIG. 66 is a diagram of the detailes of the control circuit shown inFIG. 64;

FIG. 67 is a wiring diagram of a modification of the arrangement shownin FIG. 66;

FIG. 68 is a graph similar to FIG. 65 but illustrating the arrangementshown in FIG. 67;

FIG. 69 is a longitudinal sectional view of a modification of thearrangement shown in FIG. 44 and a diagram of a control circuittherefor;

FIG. 70 is a longitudinal sectional view of a modification of thearrangement shown in FIG. 69;

FIG. 71 is a view similar to FIG. 64 but illustrating a modification ofthe arrangement shown in FIG. 64;

FIG. 72 is a view similar to FIG. 70 but illustrating a modification ofthe arrangement shown in FIG. 69; and

FIG. 73 is a longitudinal sectional view of another modification of thearrangement shown in FIG. 69.

Throughout the Figures like reference numerals designate the identicalor corresponding components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 of the drawings, there is illustrated aconventional glow discharge-heating apparatus. The arrangementillustrated comprises a hollow cylindrical cathode electrode 1, a hollowcylindrical anode electrode 2 surrounding coaxially the cathodeelectrode 1 to form an annular discharge gap 8 therebetween with the aidof two electrically insulating spacers 7 in the form of annuli fixedlydisposed between both electrodes 1 and 2 adjacent to both end portionsof the anode electrode 2, and a cylindrical enclosure 9 formed of anysuitable electrically insulating material such as glass and coaxiallyhousing the electrodes 1 and 2 with the cathode electrode 1 hermeticallyextending through both ends thereof. A seal fitting 10 is sealed at theouter periphery to one end, in this case, the lefthand end as viewed inFIG. 1 of the enclosure 9 and at the inner periphery to the adjacentportion of the cathode electrode 1 while a corrugated seal fitting 11 issealed at the outer periphery to the other end of the envelope 9 and atthe inner periphery to the adjacent portion of the cathode electrode 1.The corrugated seal fitting 11 is permitted to be axially contracted andexpanded enough to prevent the cathode electrode 1 from damaging due toan axial thermal strains thereof. Thus the envelope 9 and the sealfittings 10 and 11 maintain the discharge gap 8 hermetic.

As shown in FIG. 1, the anode electrode 2 includes flared end portions 2g in order to prevent an electric discharge from concentrating on theend portions of the anode electrode 2.

A positive terminal 5 connected to the central portion of the anodeelectrode 2 is extended and sealed through the central portion of thecylindrical peripheral wall of the enclosure 9 until it is connected toa positive side of a source of DC voltage 3 having a negative sideconnected through a stabilizing resistor 4 to a negative terminal 6 thatis, in turn, connected to one end portion, in this case, the righthandend portion as viewed in FIG. 1 of the cathode electrode 1.

In the arrangement of FIG. 1, a DC voltage is applied across the anodeand cathode electrodes 1 and 2 respectively to establish a glowdischarge across the discharge gap 8 thereby to heat the cathodeelectrode 1. Under these circumstances, a liquid to be heated such aswater is caused to flow through the interior of the cathode electrode 1to be directly heated by the heated cathode electrode 1.

Conventional glow discharge heating apparatus such as shown in FIG. 1have been enabled to instantaneously heat liquids to be heated, forexample, water resulting in heating apparatus simple in construction andstill high in efficiency. However, since the apparatus have required thehigh current, it has been extremely difficult to stably sustain the glowdischarge across the anode and cathode electrodes. According tocircumstances, there has been a fear that the glow discharge transits toan arc discharge. Further there has been a fear that, as a result oftheir heating, the electrodes are axially expanded leading to thedestruction of the heating apparatus. In addition, conventional glowdischarge heating apparatus have not been provided with suitable controlcircuit means for starting and ceasing the glow discharge with theresult that it has been difficult to reliably control the glowdischarge.

The present invention contemplates to eliminate the disadvantages of andobjections to the prior art practice as above described andcharacterized by unique means for imparting a positive resistance to thecurrent-to-voltage characteristic of the glow discharge. It has beenfound that such characteristic is effective for preventing the transitof the glow discharge to an arc discharge.

For a better understanding of the principles of the present invention,description will now be made of the glow discharge, the principles thatit heats an associated cathode electrode and the current-to-voltagecharacteristic thereof.

FIG. 2A shows a pair of cathode and anode electrodes 1 and 2respectively disposed in spaced opposite relationship and a source of DCvoltage 3 including a negative side connected to the cathode electrode 1and a positive side connected to the anode electrode 2 through astabilizing resistor 4 whereby a glow discharge occurs within adischarge space formed between both electrodes 1 and 2. It is well knownthat the discharge space having the glow discharge established thereinis divided into a region of cathode fall a in which positive ions areenriched, a region of negative glow b forming a thin lumunescent layer,a Faraday dark space c in which no light is emitted, and a positivecolumn d_(o) consisting of a plasma including electrons and ions,starting with the side of the cathode electrode 1.

FIG. 2B shows a spatial voltage profile in the discharge space with theglow discharge established therein. In FIG. 2B, a voltage V is plottedin ordinate against a distance d in abscissa measured from the cathodeelectrode 1. From FIG. 2B it is seen that the region of cathode fall ahas a very large potential-gradient because of the presence of a spacecharge until a cathode fall of potential V_(c) is reached at the end ofthe region a spaced from the surface of the cathode electrode 1 by adistance of dc. The voltage reaches a glow voltage V_(g) on the surfaceof the anode electrode 2.

By visually observing the glow discharge, it is seen that a boundarybetween the region of cathode fall a and the region of negative glow bare very distinct but a boundary between the region of negative glow band the Faraday dark space c or between the Faraday dark space c and thepositive column d_(o) is not so distinct.

Also the Faraday dark space c and the positive column d_(o) are in theso-called plasma state and relatively small in potential gradient. Onthe other hand, the region of cathode fall a includes positive ions inthe form of a beam. As far as the discharge current is concerned, itconsists essentially of an electron current in each of the Faraday darkspace c and positive column d_(o) which are in the plasma state and ofan ion current in the region of cathode fall a. The region of negativeglow b forms a region of the transition of one to the other of bothcurrents.

Two phenomena developed in the region of cathode fall a, that is, (1)the mechanism by which the glow discharge is sustained and (2) aphenomenon that the cathode electrode is heated with the glow dischargeas well as (3) the current-to-voltage characteristic of the glowdischarge are pertinent to the principles of the present invention andtherefore will now be described.

(1) Mechanism of Sustaining Glow Discharge

Positive ions present in the region of cathode fall a collide againstthe surface of the cathode electrode whereupon the cathode electrode 1emits electrons by means of the action of emitting secondary electronscalled the γ_(i) action. The electrons emitted from the cathodeelectrode (1) collide against neutral atoms or molecules during theirmovement toward the anode electrode which is accompanied by an ionizingaction called the α action with some probability. Electrons and positiveions caused by the ionization and collision are accelerated toward theanode and cathode electrodes respectively by means of the action of anelectric field involved. It is noted that the positive ions acceleratedwith the electric field contributes to the γ_(i) action.

Here the mechanism by which of the glow discharge is sustained will besomewhat quantitatively described. For example, it is said that, withthe cathode electrode 1 formed of nickel, the γ_(i) is approximatelyequal to 0.01 for slow helium ions having 1 Kev or less. That is, about100 ions collide against the cathode electrode 1 to emit a singleelectron therefrom.

Also a degree of ionization α is a function of the type and pressure ofa gas confined in the discharge space and a potential gradient developedtherein. Electron-ion pairs formed at a distance x from the cathodeelectrode 1 are proportional to e.sup.αx where e designates the base ofNapierian logarithms and therefore increase exponentially with thedistance x. Accordingly, the glow discharge is sustained with a distanceand a voltage required for about 100 electron-ion pairs to be formed inthe course of movement of a single electrode toward the anode electrode2. This distance is designated by the distance dc shown in FIG. 2B andthis voltage substantially corresponds to the voltage V_(c). In otherwords, the glow discharge can be sustained even when the anode electrode2 has been displaced to its position substantially shown by dc in FIG.2A.

This is substantially applicable to electrodes formed of nickel, copper,iron, stainless steel or the like and operatively associated with a gasselected from the group consisting of helium, neon, argon, hydrogen,nitrogen etc.

A more detailed analysis of the phenomena developed in the vicinity ofthe cathode electrode 1 teaches that a current density J on the surfaceof the cathode electrode 1 is expressed by

    J=j.sub.+ +j.sub.- =j.sub.+ =K.sub.1 P.sup.2               (1)

where j₊ and j₋ designate densities of positive ions and electronsrespectively, P a pressure of a discharge gas, and K₁ designates aconstant determined by both the type of a cathode material and that ofthe discharge gas.

Also the region of cathode fall a has a thickness dc as defined by

    d.sub.c P=K.sub.2                                          (2)

where K₂ designates a constant dependent upon both the type of thecathode material and that of the discharge gas. Within the region ofnormal glow, the cathode fall of potential V_(c) is determined by boththe type of the cathode material and that of the discharge gas butscarcely depends upon both a discharge current and the pressure of thedischarge gas.

The following Table I lists values of the constants K₁ and K₂ and thecathode fall of potential V_(c) measured within the region of normalglow with different combinations of cathode materials and dischargegases with a glow current not higher than 1 amperes and with thedischarge gases maintained under the pressure of 50 Torrs or more. Themeasured K₁ and K₂ values are expressed in 10⁻⁶ ampere per cm² Torr² andin cm.Torr and the voltage V_(c) is expressed in volts. Also the currentdensity on the surface of the cathode electrode has been determined bymeasuring an area of a negative glow b. Probably, the negative glow isvery thin so that it is observed like a luminescent film attached to thecathode electrode.

                  TABLE I    ______________________________________    MEASURED VALUES OF K.sub.1, K.sub.2 and V.sub.c                Gas    Cathode       He       Ne       Ar    H.sub.2    ______________________________________    CU        K.sub.1 6.0      8.3    27    24              K.sub.2 3.0      3.0    0.8   2.0              V.sub.c 150      150    180   290    Ni        K.sub.1 8.0      20     32    32              K.sub.2 3.0      4.0    1.5   3.0              V.sub.c 101      140    185   254    Mo        K.sub.1 4.4      4.7    17    30              K.sub.2 4.0      3.0    0.8   3.0              V.sub.c 180      175    190   290    SUS       K.sub.1 7.6      8.0    22    30              K.sub.2 5 ˜ 7                               2.5    0.8   1.5              V.sub.c 119      150    180   232    ______________________________________

(2) Heating of Cathode Electrode

As above described, positive ions present in the region of cathode falla collide against the cathode electrode to cause the γ_(i) action. Atthat time, the positive ions have surplus kinetic energy that is, inturn spent to heat the cathode electrode 1. Regarding quantities ofinput and output heat of the cathode electrodes, there are, in additionto collision with the positive ions, heat conduction from the plasmaportions, exothermic and endothermic effects caused from chemicalreactions effected on the surface of the cathode electrode 1 due to theglow discharge, cooling effects caused from the sputtering on thecathode electrode and the evaporation of the cathode material etc.However, an extent to which a quantity of heat enters the cathodeelectrode has not been elucidated until the present.

In order to determine a quantity of input heat to the cathode electrodedue to the glow discharge formed between that electrode and an anodeelectrode, experiments were conducted with a test device schematicallyshown in FIG. 3. As shown in FIG. 3, a cathode electrode 1 in the formof a very long circular rod having a radius r of 1.8 mm was disposed tobe thermally isolated from the surrounding and opposite to a similaranode electrode 2 to form therebetween a gap having a length d of 4 mm.Both electrodes were formed of copper and connected across a DC source 3through a stabilizing resistor 4. Thus a glow discharge g is establishedacross both electrodes 1 and 2 in the atmosphere. Under thesecircumstance, a radiation thermometer M was used to continuously measurea temperature at a point on the outer surface of the cathode electrode 1spaced way from the discharge surface thereof by a distance z_(o) of 3mm.

The results of the experiments are shown in FIG. 4 wherein thetemperature in Centigrade is in ordinate against time in seconds inabscissa with a glow current taken as the parameter. In FIG. 4, eachvertical segment designates a range in which measured values of thetemperature are dispersed and solid curves represent calculated valuesof the temperature as will be described hereinafter. The referencenumerals 111, 112, 113 and 114 mean the temperatures measured andcalculated with glow currents of 400, 250, 200 and 150 milliamperesrespectively.

From FIG. 4 it has been confirmed that the glow discharge transites orchanges to an arc discharge upon the measured temperature approaching1000° C. This will be because an oxide film is formed on the surface ofthe cathode electrode at such a temperature.

It is now assumed that in FIG. 3, the cathode electrode 1 with a radiumr has the longitudinal axis lying on a z axis and the discharge surfacepassing through the origin for the z axis and that a quantity of inputheat to the cathode electrode 1 is constant per unit area and per unittime. Under the assumed condition, by solving a partial differentialequation for conduction of heat referred to the z axis alone and takingaccount of a radiation loss may be expressed by ##EQU1## where κdesignates a thermal diffusibility defined by the square root of thequotient of a thermal conductivity k of the cathode electrode divided bythe product of a density ρ and a heat capacity thereof and α is aconstant on the assumption that the radiation loss is a linear functionof a temperature T. By solving the partial equation under the boundaryconditions ##STR1## where ξ designates a coefficient of heat input andthe initial condition

    T(z,o)=T.sub.o

where T_(o) designates room temperature, a solution results in ##EQU2##where I: glow current.

In the expression (3) F(γ₁) and F(γ₂) are error functions expressed by##EQU3## respectively where γ₁ and γ₂ are expressed by ##EQU4##respectively. Also α is defined by ##EQU5## where ε designates anemissivity, σ a Stefan-Boltzmann constant and Ta designates the meanvalue of room temperature and a temperature of the cathode electrode.

The expression (3) was used to calculate the time dependency of thetemperature rise on the measured point as shown in FIG. 3. The resultsof the calculations are indicated by the solid curves shown in FIG. 4.

From FIG. 4 it is seen that the measured values of the temperaturefairly well coincide with the calculated values thereof.

FIG. 5 illustrates a glow discharge voltage V in volts plotted inordinate against a length of a discharge gap in millimeters in abscissa.The voltage V was measured with the electrodes formed of copper anddisposed in the atmosphere. Curves labelled 115, 116, 117 and 118 depictglow currents of 10, 50, 100 and 400 milliamperes respectively.

In FIG. 4 it is to be noted that the curves have been drawn by equallingthe cathode drop of potential Vc in the atmosphere to a voltage of 285volts estimated with a null gap length from curves shown in FIG. 5.

Also the coefficient of heat input ξ has been determined to cause thecalculated valves of the temperature to coincide with the measuredvalues thereof shown in FIG. 4. The coefficient ξ has been of 1.4.

Further it is considered that a quantity of heat corresponding to 0.4jV_(c) per unit area per unit time will result from one portion of heatgenerated in that portion of the glow discharge formed of both theFaraday dark space c and the positive column d except for the region ofcathode fall a having a thickness dc approximately equal to 2×10⁻³centimeter.

FIG. 6 illustrates a glow voltage V_(g) in volts plotted in ordinateagainst a glow current I in milliamperes in abscissa. Curve labelled 119describes the glow current-to-voltage characteristic exhibited by thearrangement of FIG. 2. Dotted curve 120 shows the total power consumedby the glow discharge and expressed by IV_(g) while broken curve 121illustrates an electric power entering the cathode electrode andcalculated as 1.4 IV_(g). Both the glow voltage and powers in watts areplotted in ordinate against the same glow current in absussa.

From FIG. 6 it is seen that at least 80% of the total consumed powerenters the cathode electrode and that the higher the glow current I thegreater the proportion of the power entering the cathode electrode tothe total consumed power will be.

Also it is seen that a quantity of input heat q to the cathode electrode1 per unit area per unit time is give by

    q=jV.sub.c =jV.sub.g

provided that the spacing d between the cathode and anode electrodes 1and 2 respectively substantially approximates the thickness of theregion of cathode fall a (see FIG. 2), that is to say, the glowdischarge includes no plasma portion. From this it is seen that thesmaller the spacing d between the cathode and anode electrodes thelarger the proportion of the power entering the cathode electrode to thetotal consumed power will be.

FIG. 7 shows a model for a positive ion flux striking against the unitarea of the surface of the cathode electrode per unit time. In FIG. 7, asquare prism has a square bottom including each side of 1 centimeter andcontacting the surface of the cathode electrode 1 and a heightcorresponding to the velocity V_(i) cm/sec of ions multiplied by onesecond. Within the prism, positive ions designated by the symbol "crossin circle" are moved as shown at the arrow to strike or collide with thecathode electrode 1. Thus the square prism designates a positive ionflux colliding against the cathode electrode per unit area per unit timeand electrical energy of the ion flux results in the quantity of inputheat q to the cathode electrode. Since the number of the positive ionsis expressed by j/e where e designates the elementary electric chargeand since each ion has electrical energy of eV_(c), the quantity ofinput heat q is expressed by ##EQU6##

Thus the model for the positive ion flux also explains that the quantityof input heat to the cathode electrode is expressed by jV_(c) per unitarea per unit time.

From the foregoing it will be understood that the glow dischargeestablished across the cathode and anode electrodes causes the quantityof heat expressed by ξjV_(c) to enter the cathode electrode per unitarea per unit time. Also by decreasing the spacing between bothelectrodes to increase the glow current through the spacing, thequantity of input heat to the cathode electrode per unit area per unittime can approximate the product of the current density on the surfaceof the cathode electrode multiplied by the glow voltage or J·V_(g).

Therefore the glow discharge without the positive column can be utilizedas a heat source having a high efficiency because almost all heat due tothe glow discharge enters the cathode electrode and also as a heatsource having a power density variable at will by changing a gaspressure within the spacing between both electrodes because the currentdensity on the surface of the cathode electrode is proportional to thesquare of the gas pressure.

(3) Current-to-Voltage Characteristic of Glow Discharge

The current-to-voltage characteristic of the glow discharge will now bedescribed and then the principles of the present invention will bedescribed in detail.

FIG. 8 shows the relationship between a current and a voltage for theglow discharge. In FIG. 8 the axis of abscissas represents a current andthe axis of ordinates represents a voltage.

A DC voltage is applied across a cathode and an anode electrode 1 and 2respectively (see FIG. 9A) to render the anode electrode 2 positive withrespect to the cathode electrode 1 thereby to cause a glow dischargethereacross. When a current flowing through both electrodes isincreased, a negative glow region b included in the glow dischargespreads in area on the surface of the cathode electrode 1 (see FIGS. 9Aand 9B). This results in a change in current-to-voltage characteristicas shown at solid line N in FIG. 8.

However, when the current is quite low, the current-to-voltagecharacteristic droops as shown by a characteristic portion N₁ in FIG. 8.A region in which the drooping characteristic N₁ appears is called aregion of subnormal glow e.

In a region following the region of subnormal glow e an increase incurrent causes the voltage to be kept substantially constant as shown bya characteristic portion N₂ in FIG. 8 as long as that the surface of thecathode electrode 1 having the negative glow b caused thereon is smallerin area than the entire surface thereof opposite to the anode electrode2 as shown in FIG. 9A. A region in which the characteristic portion N₂is developed is called a region of normal glow f.

A further increase in current causes an increase in voltage because thenegative glow b has covered the entire area of the surface of thecathode electrode 1 opposite to the anode electrode 2 as shown in FIG.9B whereby the negative glow increases in current density. The resultingI-V characteristic is upturned with an increase in current as shown by acharacteristic portion N₃ in FIG. 8. The characteristic portion N₃ iscalled a positive resistance characteristic and a region in which thepositive resistance characteristic N₃ appers is called a region ofabnormal glow. In that region of abnormal glow g the entire area of thesurface of the cathode electrode 1 is covered with the negative glow b(see FIG. 9B) with the result that the current is apt to concentrate atthe edge portion or the like of the cathode electrode 1 and thereforethe glow discharge is easily changed to an arc discharge. As a result,it is difficult to maintain the glow discharge in its stable state. Thearc discharge appears in a region h as shown in FIG. 8.

With no impedance connected between the cathode and anode electrodes 1and 2 respectively and an electric source for supplying an electricpower across both electrodes, the source side has the current-to-voltagecharacteristic of the constant current type such as shown at horizontalbroken line P in FIG. 8. This is because even an increase in currentdoes not cause a voltage drop across an impedance.

Under these circumstance, the glow discharge has its operating pointcoinciding with a point P₁ where the characteristic P of the source sideintersects the characteristic N of the glow discharge. However, thisoperating point P₁ is located in the region of abnormal glow g, which isapt to transit to a region of arc discharge h, as above described.Accordingly it is difficult to maintain the glow discharge stable in theregion of abnormal glow g.

Further it is to be noted that the flat characteristic P of the sourceside can not stably cross the flat characteristic portion N₂ of the glowdischarge in the region of normal glow f.

On the other hand, with a resistance R as the impedance connected to thesource, an increase in current I causes an increase in voltage drop IRacross the resistance. Thus the source side has the current-to-voltagecharacteristic such as shown at dotted straight line Q in FIG. 8 and theglow discharge has its operating point designated by an intersection Q₁of the characteristics Q and N. This operating point is located in theregion of normal glow f resulting in the stable glow discharge.

Where electrical energy participating in the glow discharge is convertedto thermal energy with a very high efficiency, the connection of animpedance to the source as above described is one factor thatcontributes to decreased efficiency in utilization of electrical energy.For example, the use of a resistor causes a Joule heating loss and theuse of a reactor causes a Joule heating loss in a winding of the reactorand an eddy current loss and a hystersis loss in an iron core of thereactor. Since such energy losses scatter as thermal energy, it ispossible to recover the thermal energy. This, of course, deprives theresulting heating device of its convenience and compactness.

From the foregoing it is seen that whether or not an impedance isconnected to an electric source, the abovementioned disadvantages remainas long as the glow discharge has the current-to-voltage characteristicin the form of a curve such as shown at N in FIG. 8.

In order that the glow discharge can be maintained stable even with theflat current-to-voltage characteristic of an associated source such asshown by straight line P in FIG. 8 and without an impedance connected tothe source, the present invention includes unique means for imparting apositive resistance to the current-to-voltage characteristic of the glowdischarge in a different manner as compared with conventional abnormalglows.

First it is seen in FIG. 10A that a surface of a cathode electrode 1opposite to an anode electrode 2 has an area made sufficiently largerthan that of the anode electrode 2 so as not to impede the spread of anegative glow A b. In other words, the opposite surface area of theanode electrode 2 is limited to a small magnitude with respect to thecathode electrode. Thus a peripheral edge root b₁ of the negative glow blying on the opposite surface of the cathode electrode 1 has a distanceto the anode electrode 2 that is gradually increased as the negativeglow b spreads due to an increase in glow discharge current andtherefore a voltage across both electrodes 1 and 2 is gradually raised.Under these circumstances, the glow discharge has the current-to-voltagecharacteristic such that the voltage increases with the current as shownat broken curve T in FIG. 8. That is, the characteristic is of thepositive resistance type.

In this connection, it is to be noted that the positive resistancecharacteristic developed in the region of abnormal glow g in the priorart practice as shown at curve N₃ in FIG. 8 is cause from the fact thatthe negative glow b has spread over the surface of the cathode electrodeand can not spread any more (see FIG. 9B). Accordingly, such positiveresistance characteristic is quite different from that according to theprinciples of the present invention. As above described, the negativeglow of the present invention is permitted to sufficiently spread inresponse to an increase in current because the active surface area ofthe cathode electrode 1 opposite to the anode electrode 2 issufficiently larger than that of the anode electrode with the resultthat there is no problem that the glow discharge transits to an arcdischarge due to the impossibility of spreading the negative glow.

From the foregoing it is seen that the characteristic T of the presentinvention as shown in FIG. 8 is developed in the region of normal glowbut not in the region of abnormal glow although it has a positiveresistance.

In the present invention, even with an associated electric source havingno impedance connected thereto, therefore the characteristic T thereofintersects the characteristic of an associated source at a point T₁ (seeFIG. 8) where the glow discharge is stablized. It is to be noted thatthe point L₁ lies in the region of the normal glow unlike thecharacteristic N₃ of the prior art so that the present invention doesnot encounter the problems that the glow discharge transits to an arcdischarge and so on.

In order to impart a positive resistance to the current-to-voltagecharacteristic of the glow discharge by further increasing the distancebetween the peripheral edge b₁ of the negative glow b on a cathodeelectrode 1 and an associated anode electrode 2, the cathode electrode 1can be made cylindrical and opposite to the anode electrode 2 as shownin FIG. 10B. In the arrangement of FIG. 10B, the peripheral glow edge b₁is located on the peripheral wall surface of the cylindrical cathodeelectrode 1 at some distance from the end surface thereof. Thus, theglow edge b₁ is spaced far away from the anode electrode 2 as comparedwith the arrangement of FIG. 10A, resulting in a satisfactory positiveresistance characteristic.

When an AC voltage is applied across the cathode and anode electrodes,either electrode becomes alternately a positive electrode so that a glowdischarge is caused on the opposite surface of both electrodes. With anAC voltage used, it is desirable that the cathode and anode electrodesare in the form of identical cylinders and oppose to each other as shownin FIG. 10C. From FIG. 10C it is seen that the peripheral edge b₁ of thenegative glow b on either electrode 1 or 2 is spaced far away from theother electrode 2 or 1 as in the arrangement of FIG. 10B.

From the foregoing the following can be summarized that the principlesof the present invention are to make an electrode area with which a pairof cathode and anode electrodes are opposite to or confront each otherto be smaller than that area of the electrode on which a negative glowis caused.

Referring now to FIG. 11, there is illustrated one embodiment accordingto the glow discharge heating apparatus embodying the principles of thepresent invention as above described. The arrangement illustratedcomprises an electrically insulating enclosure 9 in the form of a hollowcylinder formed of glass, a cathode electrode 1 in the form of a hollowcylinder with both open ends coaxially extending through the enclosure 9and an anode electrode 2 in the form of a hollow cylinder with both openflare ends disposed coaxially with the cathode electrode 1 withing theenclosure 9 to form an annular glow discharge gap 8 therebetween. Thecathode electrode 1 is extended and sealed through both ends of theenclosure 9 by means of seal fittings 10 and 11 respectively. Thus theenclosure 9 along with the cathode electrode 1 defines an annular space81 which includes the glow discharge gap 8 and is filled with anelectrically dischargeable gas selected from the group consisting ofrare gases such as helium, mixtures thereof, for example, a mixture ofneon and argon, a mixture of helium and hydrogen etc.

An annular anode terminal 5 is fixedly secured at the inner periphery tothe central portion of the outer cylindrical surface of the anodeelectrode 2 and has a protrusion extended and sealed through theenclosure 9 by having the outer periphery fixed to a seal fitting sealedto adjacent ends of two similar enclosure portions forming the enclosure9. The anode terminal 5 is connected to a positive side of a source ofDC voltage 3 including a negative side connected by a stabilizer 4 to acathode terminal 6 that is connected to that portion of the cathodeelectrode 1 disposed outside of the enclosure 9, in this case, adjacentto the seal fitting 11. The stabilizer 4 may be a small capacity reactoror a resistor. If desired, the stabilizer may be omitted.

In order to facilitate the description of the present invention, thesymbol "S-" designates the entire area of that portion of the cathodeelectrode 1 on which a glow discharge can be caused while the symbol"S+" designates an area of that portion of the anode electrode 2opposing the cathode electrode 1 and actually used for the glowdischarge. Therefore an area labelled S+ is called an "anode areaeffective for discharge" or an "effective anode area".

According to the principles of the present invention as above described,the anode area S+ effective for discharge is made smaller than thecathode discharge area S-.

The operation of the arrangement as shown in FIG. 11 will now bedescribed. A DC voltage from the source 3 is applied across the anodeand cathode electrodes 2 and 1 respectively through the stabilizer 4 toestablish a stable glow discharge in the annular discharge gas 8 therebyto heat the cathode electrode 1. Under these circumstances, a fluid tobe heated, for example, water flows into the interior of the cathodeelectrode 1 as shown at the arrow A in FIG. 11 to absorb heat from thecathode electrode 1 to be heated. Then the heated fluid flows out fromthe cathode electrode 1 as shown at the arrow B in FIG. 1.

During the glow discharge, a current and a voltage thereof isillustrated by a characteristic curve shown in FIG. 12 wherein the glowdischarge current I_(g) in amperes is plotted in abscissa against theglow discharge voltage V_(g) in volts in ordinate. The glow dischargevoltage V_(g) may be approximately expressed by

    V.sub.g =V.sub.o +I.sub.g R

where V_(o) designates a glow discharge-hold minimum voltage as will bedescribed hereinafter and R designates a slope of the characteristiccurve.The slope of the characteristic curve as shown in FIG. 12 iscalled a "positive resistance R".

Referring back to FIG. 11, L designates an axial length of the anodeelectrode 12 and has been changed to vary the effective anode area S+thereby to obtain the relationship between a ratio of the effectiveanode area S+ to the cathode discharge area S- and the positiveresistance R as shown in FIG. 13.

In FIG. 13, the positive resistance R in ohms is plotted in ordinateagainst the ratio between both area S+/S- in abscissa. Curves labelled122, 123 and 124 have been plotted with data measured by filling theinterior of the enclosure 9 or the annular space 81 with a gaseousmixture including 70% by volume of helium and 30% by volume of hydrogenunder pressures of 100, 150 and 200 Torrs respectively. The gap betweenboth electrodes 1 and 2 was maintained at a magnitude of 1 mm. Also thevertical segment has the same meaning as that shown in FIG. 4.

From FIG. 13 it is seen that the positive resistance R at the ratio ofS+/S- of 0.2 increases to four or five times that at the ratio of 1.

The tendency of the positive resistance characteristic as shown in FIG.13 can be observed with the spacing of 5 mm between the electrodes 1 and2 filled with the dischargeable gas including neon, helium, a mixture ofneon and argon, or a mixture of helium and at most 30% by volume ofhydrogen under a pressure of 200 Torrs or less.

Also experiments have been conducted with the DC source 3 having variedregulations of the source voltage.

The results of the experiments are shown in FIG. 14 wherein the axis ofordinates represents a regulation of source voltage in percent and theaxis of abscissas represents a ratio of the actual discharge current Ito a rated discharge current I_(o) in percent. Straight lines labelled125 and 126 describe the regulations of source voltage with the positiveresistance R having values of 1 and 3 ohms respectively.

From FIG. 14 it is seen that a variation of 15% in source voltage givesa current regulation or a ratio of the actual current I to the rateddischarge current I_(o) multipled by one hundred in percent ±42% and±14% with positive resistance R of 1 and 3 ohms respectively. Thus thepositive resistance R of 3 ohms renders the glow discharge relativelystable.

Further by rendering the positive value R higher, it is possible tocontrol a maximum current for supplying a predetermined electric powerto a small magnitude which is, in turn, advantageous in that the glowdischarge apparatus is made compact.

The measure as above described is also applied to constructions in whichAC voltage is applied across the electrodes 1 and 2 to cause the glowdischarge thereacross only when the electrode 1 acts as a cathodeelectrode.

From the foregoing it is seen that the arrangement of FIG. 11 eliminatesthe disadvantages of conventional glow discharge heating apparatus thatthe positive resistance for the glow discharge is low, the glowdischarge is moved about on the electrode, the discharge current greatlychanges with a variation in source voltage resulting in the necessity ofconnecting a stabilizer or the like to the source and so on. Thosedisadvantages have been caused from the cathode area being substantiallyequal to the anode area.

FIG. 15 shows a modification of the present invention operativelyassociated with an AC source. The arrangement illustrated comprises aninner electrode 1 in the form of a hollow cylinder having one endclosed, an outer electrode 2 in the form of a hollow cylinder having oneend open and disposed coaxially with the inner electrode 1 so that theclosed end portion of the inner electrode 1 is inserted into the openedend portion of the outer electrode 2 to form an annular discharge gap 8therebetween.

The inner electrode 1 is coaxially disposed within a tubular glassenclosure 9 to extend beyond both open ends thereof and the open endportion of the electrode 1 is rigidly fitted into an annular supportingdisc 13 of any suitable metallic material including an outer peripheryconnected to the adjacent end of the enclosure 9 through the sealfitting 11. The outer electrode 2 has the open end portion extendinginto the enclosure 9 and supported by another annular supporting disc 14of the same material as the disc 13 similarly connected to the other endof the enclosure 9 through another seal fitting 10. In this way theenclosure 9 defines a hermetic space 81 with the supporting discs 13 and14, the seal fittings 10 and 11, the inner electrode 1 and the outerelectrode 2 having the other end closed.

Then a pair of terminals 5 and 6 is attached to the supporting discs 13and 14 to connect both electrodes 1 and 2 to an AC source 31therethrough.

An inflow tube 15 is coaxially disposed within the inner hollowelectrode 1 to form an annular passageway therebetween. The tube 15 ismaintained in place through a closing member 16 rigidly fitted into theopen end of the inner electrode 1 and having the tube 15 extendingtherethrough. The inner electrode 1 is provided on the open end portionwith an outlet duct 17.

On the other hand, the outer electrode 2 is double walled and providedon the closed end portion of the outer wall with an inlet duct 18 andthat portion thereof adjacent to the supporting disc 14 with an outletduct 19 communicating with the inlet duct 18 through an annular spacedefined by the inner and outer walls of the electrode 2. A liquid to beheated, for example, water enters the inlet duct 18 as shown at thearrow A in FIG. 15 and thence to the annular space due to thedouble-walled structure of the outer electrode 2 after which it leavethe outlet duct 19. Also water enters the inflow tube 15 as shown at thearrow C in FIG. 15 and thence the annular space between the inflow tube15 and the inner electrode 1. Then the water flows out from the outletduct 17 as shown at the arrow D in FIG. 15.

It will readily be understood that the space 81 is filled with an easydischargeable gas as above described in conjunction with FIG. 11.

In operation an AC voltage across the source 31 is applied across bothelectrodes 1 and 2 to cause a glow discharge mainly in the annulardischarge gap 8.

As above described, the inner electrode 1 is inserted into the outerelectrode 2 to overlap the latter. This ensures that an area of thatportion of one of the electrodes opposite to the other electrode issmaller than an electrode area with which a glow discharge can occurbetween the electrodes 1 and 2. This means that an anode area on theside of that electrode acting as an anode for the glow discharge isalways limited. For example, with the dischargeable gas maintained undera pressure of about 200 Torrs and with the gap between both electrodeshaving a length not exceeding 5 millimeters, the limitation of the anodearea results in an indirect limitation of an associated negative glowregion and therefore an increase in positive resistance for the glowdischarge. That is, the current-to-voltage characteristic of the glowdischarge such as shown at curve T₁ in FIG. 8 has a larger slope wherebythe AC glow discharge can be maintained stable. Accordingly, a stableglow discharge can be sustained even with a high current under a highpressure without the glow discharge being changed to an arc discharge.

Under these circumstances, either of the inner and outer electrodes 1and 2 respectively is heated when it acts as the cathode electroderesulting in heating of both electrodes. Thus the fluid such as waterflowing in contact with the electrodes is instantaneously heated and theheated fluid leaves the outlet ducts 17 and 19.

FIG. 16 is a characteristic curve illustrating the relationship betweenthe area of one of the electrodes overlapping the other electrode andthe positive resistance exhibited by the glow discharge. In FIG. 16 thepositive resistance R in ohms is plotted in ordinate against a ratio ofthe overlapping area to the entire area of the electrode acting as thecathode in abscessa. Curves labelled 127, 128 and 129 have been plottedwith the discharge gap 8 having a length not exceeding 5 mm and filledwith a mixture of helium and hydrogen under pressures of 100, 150 and200 Torrs respectively. The vertical segment has the same meaning asthat shown in FIG. 4.

From FIG. 16 it is seen that the smaller the overlapping area for bothelectrodes 1 and 2 the higher the positive resistance will be.

In the arrangement of FIG. 15, the inner and outer electrodes 1 and 2respectively are disposed in coaxial relationship but different in shapefrom each other. Therefore the current-to-voltage characteristic of theglow discharge is different between the half-cycle of the source 31having the inner electrode 1 acting as a cathode and that having theouter electrode acting as an anode as shown in FIG. 17. In FIG. 17, theaxis of ordinates represents a discharge voltage V and the axis ofabscissas represents a discharge current I. When the inner electrode 1acts as the cathode, the discharge current I is forwardly and rearwardlychanged along a straight line 130 shown in FIG. 17 and has a maximumvalue of I₁. In the next succeeding half-cycle the outer electrode 2takes over the cathode and the current is forwardly and rearwardlychanged along a straight line 131 shown in FIG. 17. In the latter case,the current has the absolute maximum value I₂ different from that of thecurrent I₁ flowing in the just preceding half cycle of the source 31.Both straight lines have the same absolute values of a voltage V_(o) ata null current. Thus the resulting characteristic becomes unsymmetric topermit a zero-phase sequence component of a current to flow through theAC source 31. This is objectionable to the source 31. Further the innerelectrode 1 is free at one end but the outer electrode 2 includes nofree end. This results in the occurrence of thermal strains in the outerelectrode 2 during the glow discharge.

These objections can be eliminated by still another modification of thepresent invention shown in FIG. 18. In the arrangement illustrated, afirst electrode 1 in the form of a hollow cylinder having one end closedwith a flat disc opposes a second electrode 2 identical to the firstelectrode to form a discharge gap 8 having a predetermined spacing orgap length of d between the opposite closed end surfaces.

A flow confining tube 20 or 21 of the double wall type inserted into thesecond or first electrode 2 or 1 respectively includes a central tubularportion extending on the longitudinal axis of the correspondingelectrode, a radially extended end wall to form a predetermined gapbetween the same and the internal closed end surface of the electrodeand a peripheral wall extending in parallel to the internal peripheralsurface of the latter to form also a predetermined annular gaptherebetween. Each electrode 1 or 2 is provided on the open end portionwith an outlet duct 18 or 17 communicating with the flow path formedtherein while annular blind cover disc 23 or 22 is rigidly inserted intothe annular gap between the peripheral surface of the electrode 1 or 2and the outerwall of the tube 21 or 20 at the open end. The purpose ofthe flow confining tubes 20 or 21 is to cause a fluid to be heated toenter first the central tubular portion as shown at the arrow A or C inFIG. 18 and flow along the internal surface of the correspondingelectrodes at an increased speed to enhance the heat transfer betweenthe fluid and the electrode and also to enable the fluid to beinstantaneously heated. The heated fluid then flows out from the outletduct 18 or 17 as shown at the arrow B or D in FIG. 18.

Then the first and second electrodes 1 and 2 respectively are snuglyfitted into individual supporting rings 14 and 13 which are hermeticallyconnected to both ends of circular enclosure 9 through annular sealfittings 10 and 11. In this way both electrodes 1 and 2 are supported ina cantilever manner to the supporting members 14 and 13 and thesubstantiall portions thereof are coaxially disposed within theenclosure 9 to form the space 81 that is then filled with adischargeable gas such as previously described.

As in the arrangement shown in FIG. 11 or 15, the AC source 31 isconnected across the electrodes 1 and 2 through the terminals 6 and 5connected thereto respectively.

In the arrangement of FIG. 18 it is noted that those portions of bothelectrodes 1 and 2 superposing each other as designated by the referencecharacter 1 is made smaller in area than that portion of each electrodeon which the glow discharge occurs. In the example illustrated the glowdischarge occurs on each of the electrodes 1 and 2 throughout thesurface.

The arrangement of FIG. 18 is characterized in that the electrodes 1 and2 formed to be symmetric face each other with the predetermined gap 8formed therebetween. This results in a symmetric glow dischargecharacteristic as shown in FIG. 19. In FIG. 19 similar to FIG. 17, thecharacteristics 132 and 133 are substantially symmetric and haverespective discharge currents I₁ and I₂ equal in absolute value to eachother.

Also, as the electrodes 1 and 2 are supported in a cantilever manner tothe annular supporting discs 14 and 13 respectively, the electrodes areprevented from breaking due to thermal stains.

It will readily be understood that the gap 8 between both electrodes 1and 2 should be dimensioned so that the electrodes are prevented fromcontacting each other due to thermal expansions thereof in operation.

As in the arrangement of FIG. 15, an AC voltage across the source 31 isapplied across the electrodes 1 and 2 to cause a glow discharge betweenthe opposite surfaces thereof while a fluid to be heated enters theinteriors of the electrodes 1 and 2 as shown at the arrows A and C inFIG. 18. Then the fluid flows through spacing formed between eachelectrode and the flow confining tube 21 or 22 to be heated with heatgenerated on the electrode 1 or 2 due to the glow discharge. Thereafterthe heated fluid flows out from each outlet duct 19 or 17.

FIG. 20 illustrates a modification of the arrangement shown in FIG. 18.As shown in vertical section in FIG. 20A, the electrodes 1 and 2 haveidentical structure and are opposed somewhat offset from each other toform a predetermined discharge gap 8 therebetween. As seen in sideelevational views of FIGS. 20B and 20C, the electrodes 1 and 2 are inthe form of rectangular boxes and therefore discharge surfaces thereofare rectangular and flat. Then each electrode is provided on the rearsurface with a pair of inlet and outlet tubes.

In other respects, the arrangement is substantially identical to thatshown in FIG. 18. The electrodes 1 and 2 include the discharge surfacesidentical in shape to each other and are of the cantilever type so thatthe arrangement exhibits the same results as that shown in FIG. 18.

In the arrangements of the present invention shown in FIGS. 15, 18 and20 the electrode material and impurities such as metallic oxidesincluded in the electrodes might be scattered in the discharge gapduring the glow discharge and stick to that surface portions of theenclosure 9 facing the electrodes 1 and 2. This sticking of suchmetallic materials to the enclosure might lead to not only a danger thatthe seal fitting 10 and 11 are short-circuit with each other through thestuck electrode materials and impurities but also to a fear that, if thescattered impurities again adhere to the electrodes that the glowdischarge will have transited to an arc discharge.

The present invention also contemplates to eliminate the danger and fearas above described, by the provision of the arrangement shown in FIG.21. The arrangement illustrated is different from that shown in FIG. 18only in that in FIG. 21 a pair of annular shields 24 and 25 one for eachelectrode, are disposed to surround the corresponding electrodes andface at least the internal surface portions of the enclosure 9 by havingflare ends thereof fixedly secured to the internal surface portions ofthe enclosure 9 respectively. Each shield 24 or 25 includes asubstantial portion parallel to the associated electrode and endingshort of the adjacent annular supporting disc 13 or 14. The shields 24and 25 may be of an electrically insulating or conductive material.

In operation when the electrode material and the impurities are emittedfrom the electrode 1 or 2 and scattered in the discharge gap, they stickto that surface of each shield 24 or 25 facing the associated electrodeand are prevented from adhering to that inner surface portion of theenclosure 9 covered with the shield 24 or 25. Also the shield iseffective for preventing the scattered electrode material and impuritiesfrom again adhering to the associated electrode.

The arrangement illustrated in FIG. 22 is different from that shown inFIG. 21 only in that in FIG. 22 a pair of annular electrodes 26 and 27are buried in the annular shields 24 and 25 formed of an electricallyinsulating material respectively. Then a suitable voltage is applied tothe annular electrode 26 and 27 whereby the scattered metallic materialsare apt to adhere to the shields 24 and 25.

FIG. 23 shows another modification of the arrangement illustrated inFIG. 21. In FIG. 23 the electrodes 1 and 2 are in the form of hollowflat discs and disposed in opposite relationship to form the dischargegap 8 having a predetermined gap length of d therebetween.

The seal fitting 10 in the form of a short hollow cylinder has one endfixedly secured to the peripheral portion of that surface of theelectrode 1 remote from the electrode 2 and the other end in the form ofa flange to an enclosure portion 91 in the form of an annulus. Then anannular shield disc 28 of electrically insulating material is locatedbetween the annular enclosure portion 91 and the peripheral portion ofthe electrode 1 by having a fitting perpendicular to the same andconnected to the outer peripheral surface of the seal fitting 10. Thesealing fitting 11, an enclosure portion 92 and a shield 29 identical tothe components 10, 91 and 28 respectively are operatively coupled in thesame manner to the electrode 2.

A toroidal metallic enclosure portion 93 of double L-shaped crosssection is hermetically connected to the annular enclosure portions 91and 92 to form a hermetically closed space 81 in the form of a toroid.

As shown in FIG. 23, a feed water tube 18 and a drain tube 19 project inspaced relationship from that surface of the electrode 1 remote from theelectrode 2 and a pair of deflector or baffle plates 30 and 32 aredisposed in the interior of the electrode 1 so as to direct a liquid tobe heated toward the peripheral portion thereof and enter the fluid intothe drain tube 19 after it has flowed along the heated surface of theelectrode 1 to be heated. Also a feed water tube 18' and a drain tube 17similarly project from the electrode 2 and a pair of baffle plates 33and 34 are similarly disposed within the hollow electrode 2.

If desired, the shield 28 and 29 may be formed of any suitable metallicmaterial. In the latter case, the shields 28 and 29 are suitablyinsulated from the associated electrodes 1 and 2 respectively.

Further the present invention contemplates to prevent the occurrence ofelectric shock-accidents through the heated liquid such as water.

The arrangement illustrated in FIG. 24 is substantially similar to thatshown in FIG. 22 except for the provision of means for preventing theuser from receiving electric shocks. As shown in FIG. 24, the controltubular portion of the flow confining tube 20 or 21 is connected to anelectrically insulating tube 37 or 38 that is, in turn, connected tometallic inflow tube 41 or 42.

The outlet of the flow confining tube 20 or 21 is connected toconnecting tube 35 or 36 subsequently connected to an electricallyinsulating tube 39 or 40 that is, in turn, connected to a metallicoutflow tube 43 or 44.

The metallic tubes 41 and 43 are electrically connected together toground as do the metallic tube 42 and 44.

It has been found that an end-to-end distance l_(p) between the centraltubular portion of the flow confining tube and the inflow tube orbetween the connecting tube and the outflow tube, that is to say, alength of the insulating portion should be equal to or less than apredetermined magnitude dependent upon a voltage applied across theelectrodes, a resistivity of the particular liquid to be heated, a crosssectional area of the tube etc.

The arrangement of FIG. 24 is operated as follows: A switch 45 is closedto apply an AC voltage from the source 31 across the electrodes 1 and 2.This causes the flow confining tubes 20 and 21, and the connecting tubes35 and 36 to be put at a certain potential relative to the groundpotential. For example, in glow discharge heating apparatus having adischarge input of about 8 kilowatts, the AC source 31 is required tosupply to the heating apparatus an AC voltage having the effective valueof 200 volts so that the tubes 20, 21, 35 and 36 are put at a voltagehaving the effective value of 200 volts.

On the other hand, the metallic inflow tubes 41 and 42 and the metallicoutflow tubes 43 and 44 are connected to ground so that the particularliquid flowing into or out from the extremities thereof is put at a nullpotential. This ensures that electric shock accidents are prevented fromoccurring through the liquid.

More specifically, the source voltage is applied across the electrodes 1and 2 to cause a glow discharge therebetween. Heat generated during theglow discharge heats the liquid. When the heated liquid flows within theapparatus, the same reaches any of the tubes 41, 42, 43 and 44 where itis put at the ground potential. This ensures that the user is maintainedsafe.

Under these circumstances the electrodes 1 and 2 rapidly transfers heatto the liquid flowing within the interiors thereof to prevent theelectrodes 1 and 2 from effecting an abnormal temperature rise wherebythe stable glow discharge is sustained.

However, as a potential difference having the effective value of 200volts occurs between the inflow and outflow tubes 41, 42 and 43, 44 andthe confining and connecting tubes 20, 21 and 35, 36, the insulatingtubes 37, 38, 39 and 40 must have a dielectric strength for withstandinga voltage having the effective evalue of 200 volts. In this connection,it is required to consider a leakage current flowing to ground throughthe liquid, in addition to the surface status of the insulating tubes.

In the arrangement of FIG. 24 applied to a water warmer operated withthe source voltage of 200 volts, the same is obligated to be providedwith a leakage breaker. Leakage breakers are responsive to the leakagecurrent in excess of the predetermined magnitude flowing through theinflow and outflow tubes 41, 42 and 43, 44 to ground to be continuouslyoperated to prevent the source voltage from being applied across theelectrodes 1 and 2. Accordingly, it is required to impart to the lengthl_(p) of the insulating portion a value sufficient to limit the leakagecurrent to a certain value or less.

Assuming that each of the insulating tubes 37, 38, 39 and 40 has a crosssectional area of flow path designated by S and a liquid to be heatedsuch as water has a resistivity designated by ρ, the insulating portionpresents a resistance R_(l) before the liquid expressed by ##EQU7## Alsoassuming that each of the insulating tubes 37, 38, 39 and 40 has asurface resistance sufficiently large as compared with the resistance ofthe liquid, the leakage current I_(l) may be expressed by ##EQU8## whereV_(l) designates a voltage across the liquid located in the insulatingportion having the length of l_(p). Accordingly, the leakage currentI_(l) is inversely proportional to the length l_(p) with the voltageV_(l), the cross sectional area S and the resistivity ρ remainingunchanged.

FIG. 25 a graph illustrating the relationship between the length l_(p)of the insulating portion and the leakage current I_(l) on the basis ofthe above two expressions (4) and (5) and with V_(l) =200 volts, S=0.636square centimeters (which results from the insulating tubes 37, 38, 39and 40 having the inside diameter of 9 millimeters) and ρ=1300ohms-centimeter. The resistivity of 1300 ohms-centimeter is a minimumvalue of a resistivity of usable water as determined by the IECstandards. In FIG. 25 the leakage current I_(l) in milliamperes isplotted in ordinate against the length l_(p) of the insulating portionin centimeters in abscissa.

Assuming that the particular water warmer is provided with a highlysensitive leakage breaker having a rated sensible current of 15milliamperes, the breaker has a rated inoperative current of 7.5milliamperes. In order to prevent this leakage breaker from beingcontinuously operated due to a leakage current flowing through theinsulating portion, the length l_(p) of the latter is necessarily of atleast 13 centimeters with used water having a resistivity of 1,300 ohmscentimeter as will be seen from the curve of FIG. 25. The expression (5)indicates that the length l_(p) changes with the leakage current,voltage, the cross sectional area of the flow path and resistivity ofthe liquid. However, a length of the particular insulating portion canbe estimated as above described and in accordance with the rating of agiven leakage breaker, the source voltage, a resistivity of theparticular liquid and the cross sectional area of the flow path.

In the arrangement of FIG. 24, the flow path of the heated liquid hasbeen provided with the insulating tubes having the required length whileeach of the insulating tubes has been connected at the extremity to themetallic inflow or outflow tube that is connected to ground.Accordingly, it is ensured that any electric shock accident can beprevented from occurring through a liquid involved and still one caneliminate the insulating treatment in which electrode components arecoated with an electrically insulating material. This results insimplified inexpensive apparatus and also the heat transfer from theelectrode components to the liquid being rapidly effected. Therefore thearrangement of FIG. 24 is extremely advantageous in both the heatefficiency and the stability of operation.

Also glow discharge heating apparatus such as shown in FIG. 24 can beutilized to instantaneously heat a liquid, for example, water by flowingthe water in a flow rate of from 1 to 10 liters per minute through theinterior of the electrodes thereby to transfer thermal energy injectedinto the electrodes to the water. Under these circumstances, water atroom temperature must be heated to a temperature of about 80° C. Thisresults in the necessity of injecting thermal energy of at least 5kilowatts into the electrodes. This means that, with a power source ofAC 200 volts used, the effective current of at least 25 amperes mustflow through the electrodes. If a discharge current becomes high andalso if the discharge gap is filled with a gas under an increasingpressure then it is difficult to sustain the flow discharge. Forexample, the glow discharge transits to an arc discharge.

It has been found that the stable maintenance of the glow discharge isaffected by the type of gas filling the discharge space. Also it hasbeen experimentally confirmed that, by filling the discharge space witha mixture of at least helium and hydrogen, the glow discharge can besustained without the transit to an arc discharge, even with an electricpower required for heating the particular liquid, that is to say, adischarge current as high as possible.

This will now be described in conjunction with FIG. 24. Variousexperiments were conducted with the discharge space 81 filled with aninert gas heavier than argon under a pressure ranging from 50 to 200Torrs. The result of experiments indicates that the glow discharge isdifficult to spread and that an increase in glow current causes acontraction of a positive column included in the glow discharge to movethe glow discharge about on the electrodes 1 and 2. Thus the glowdischarge is put in its unstable state so that it is apt to transit toan arc discharge. The mean value of the glow current in excess of 5amperes has caused the glow discharge to transit to an arc discharge.

With neon used, relatively stable glow discharge has occurred under agas pressure not higher than 70 Torrs. Under a gas pressure of 100Torrs, however, the glow discharge has been relatively stable at aneffective current up to about 20 amperes. Upon the effective currentexceeding 20 ampere, the positive column has been contracted. This mightcause the glow discharge to transit to an arc discharge.

Further, when an inert gas used has been heavier than neon, the scatterfrom the electrodes 1 and 2 has increased in amounts with the resultthat the electrodes 1 and 2 are violently consumed while insulatingmaterials such as glass forming the enclosure 9 is sharply deterioratedin electrical insulation qualities because metallic materials scatteredfrom the electrodes 1 and 2 stick thereto. As a result, the useful lifeof the glow discharge heating apparatus has been greatly reduced.

From the foregoing it is summerized that, with the arrangement of FIG.24 used as a heating apparatus for instantaneously heating water, it isrequired to sustain stably the glow discharge under a relatively highpressure of 50 Torrs or more and still at a high current exceding 25amperes at an AC voltage of 200 volts.

Also from the foregoing it has been found that it is desirable to fillthe discharge space 81 with a chemically stable, light inert gas andsuitable examples of the inert gas involve helium and hydrogen.

In the arrangement of FIG. 24, however, it has been seen that, withhelium filling the discharge space 81, the flow discharge spreadsthroughout the surface of the electrodes 1 and 2 at low current becauseof a small current density and that electrical energy of the glowdischarge entering the electrodes 1 and 2 amounts only to about 2Killowatts. Also in a glow discharge caused in an atmosphere of helium,its positive column has been contracted upon a pressure of heliumincreasing to 150 Torrs to increase a current density for the glowdischarge. Thus the glow discharge has been moved about on theelectrodes and become unstable. The glow discharge will often transit toan arc discharge.

On the other hand, a glow discharge in an atmosphere of hydrogen hasmade a discharge hold minimum voltage Vo equal to at least 240 volts asshown in FIGS. 30, 31 and 32 which will be described hereinafter.Therefore, it has been difficult to cause a glow discharge having anelectric power of 5 kilowatts or more by using an AC source with 200volts.

It has been found that, in order to manufacture glow discharge heatingapparatus requiring at least 5 kilowatts with an AC voltage of 200volts, it is optimum to employ a mixture of helium (He) and hydrogen(H₂) as a filling gas.

When the arrangement of FIG. 24 is filled with a mixture of helium andhydrogen under a pressure of 100 Torrs, and applied with an AC voltageof 60 Hertz having a waveform E shown in FIG. 26, a glow current flowingtherethrough is changed in accordance with a proportion of hydrogen tohelium as shown at current waveforms F, G, H and I in FIG. 26. FIG. 26shows the voltage and current waveforms in one cycle of the sourcevoltage. The current waveforms F, G, H and I have been plotted for agaseous mixture including 5, 10 30 and 50% by volume of hydrogen and thebalance, helium respectively.

Also the glow discharge exhibits the current-to-voltage characteristicdependent upon the proportion of the hydrogen to the helium as shown inFIG. 27 wherein a voltage in volts is plotted in ordinate against acurrent I in amperes in abscissa and like reference characters have beenemployed to identify the helium-hydrogen mixtures identical to thosedesignated in FIG. 26. As shown in FIG. 27, each of thecurrent-to-voltage characteristics is substantially rectilinear. Bycalculating both values of glow voltages S, T, U and W through theextrapolation and slopes of respective characteristic curves, the glowvoltage Vg may be approximately expressed by

    Vg=V.sub.o +RI

where V_(o) designates a glow discharge hold minimum voltage designatedby S, T, U or W, and R designates the slope of the characteristic calledthe positive characteristic R. As is well known, the voltage V_(o) isexpressed by V_(o) =Em sin ωt where Em designates the peak value thereofand ω designates an angular frequency of the source voltage. Tocalculate a discharge power P from the above expression for V_(o)referring to FIG. 26 gives ##EQU9## where T designates a period of thesource voltage. The discharge voltage is thermal energy entering theelectrodes 1 and 2 due to the glow discharge.

Assuming that the source voltage has its frequency of 60 hertzs and 200volts or the peak value of Em=√2 200≃280 volts, its period is of 16.67milliseconds and its angular frequency is of 377 radius per second. Byusing those figures in the expression for the discharge power, the glowdischarge hold minimum voltage V_(o) relates to the positivecharacteristic R as shown in FIG. 28 wherein the positive resistance Rin ohms is plotted in ordinate against the glow hold minimum voltageV_(o) in volts in abscissa with the parameter being the discharge poweror thermal energy P.

From the FIG. 28 it is seen that, in order to provide the thermal energynot less than 5 kilowatts, the V_(o) and R may lie in a hatched regionas shown in FIG. 28 defined by a line for the power of 5 kilowatts, andboth coordinate axes.

Also the glow hold minimum voltage V_(o) is determined by a pressure ofa filling gas and the gap length d between the electrodes 1 and 2 whilethe positive characteristic R is determined by the configuration of theelectrodes of the overlapping area S_(o) for both electrodes 1 and 2 andthe pressure of the filling gas.

By changing a relative diameter M of one to the other of the electrodes1 and 2 to vary the overlapping area S_(o) therefor and also by changingthe pressure of the filling gas, the positive characteristic R is variedas shown in FIG. 29 wherein the overlapping area S_(o) in squarecentimaters is plotted in ordinate against the pressure of the fillinggas in Torrs in abscissa with the positive characteristic R variouslychanged. In FIG. 29 solid line indicates measured values and dotted lineindicates values estimated from the associated measured values.

From FIG. 29 it is seen that, under a gas pressure less than 50 Torrs, acurrent density for the glow discharge is low and the supply of adischarge power or a heat input in excess of 5 kilowatts to theelectrodes requires an increase in overlapping area S. This hasencountered a problem with apparatus portability because the electrodearea must increase.

On the other hand, a gas pressure in excess of 150 Torrs causes thedischarge input to the electrodes to increase to at least 5 kilowatts,resulting in a glow current of at least 25 amperes. Under thesecircumstances a positive column involved is contracted and theparticular glow discharge is moved about on the electrodes. This mightsometimes cause the glow discharge to transit to an arc discharge.

With the gas pressure further increased to 200 Torrs or higher, apositive column involved is contracted at a glow current of at least 5amperes until the transit to an arc discharge occurs.

As an example, it is assumed that it is impossible to decrease the glowhold minimum voltage V_(o) to 176 volts or less. Under the assumedcondition, it is seen from FIG. 28 that, in order to manufacture glowdischarge heating apparatus having a discharge input of at least 5kilowatts, the pressure of the particular filling gas, the overlappingarea S_(o) and the positive characteristic R must lie in the hatchedportion shown in FIG. 29 as being defined by a pair of vertical brokenlines passing through the abscissas of 50 and 150 Torrs respectively andcurve labelled R=2Ω.

In addition, by changing both the proportion of hydrogen to helium andthe gap length d between the electrodes 1 and 2, the glow hold minimumvoltage V_(o) is varied as shown in FIGS. 30, 31 and 32 wherein the axisof ordinates represents the proportion of hydrogen to helium in percentand the axis of abscissas represents the gap length d in millimeters.The helium-hydrogen mixture is maintained under pressures of 50, 100 and150 Torrs in FIGS. 30, 31 and 32 respectively. In these Figures thecurves are labelled with the measured values of the glow hold minimumvoltage V_(o) and for pure hydrogen the measured voltages V_(o) aredenoted aside corresponding dots.

Also the gap length d less than about 0.5 millimeter between bothelectrodes 1 and (2) has resulted in a danger that both electrodes maycontact and shortcircuit due to a pressure difference between a pressureof the particular heated liquid within either of the electrodes and thatof a filling gas involved. On the other hand, an excessively large gaplength d between both electrodes cause a positive column to constract tomove the resulting discharge about on the electrodes until the dischargesometimes transits to an arc discharge. This might result in damage tothe electrodes 1 and 2. It has been seen that the contraction of thepositive column occurs with the gap length d of at least 9, 6 and 3millimeters under the gas pressures of 50, 100 and 150 Torrsrespectively.

With the proportion of hydrogen to helium decreased to 2.5% or less, theresulting glow discharge resembles that occurring in an atmosphere ofpure helium. This has made it difficult to increase the discharge inputto at least 5 kilowatts. Also as FIG. 29 illustrates it is difficult todecrease the positive characteristic R to at most 1, 0.5 and 0.3 ohmsunder gas pressures of 50, 100 and 150 Torrs respectively, and it hasbeen difficult to increase the discharge input to at least 5 kilowattsat the glow hold minimum voltages V_(o) of at least 210, 230 and 240volts under the gas pressures of 50, 100 and 150 Torrs respectively aswill readily be understood from the graph shown in FIG. 28. Further anincrease in glow hold minimum voltage V_(o) causes an increase in peakvalue of the glow current as shown in FIG. 33 wherein the peak currentfor the glow discharge in amperes is plotted in ordinate against theglow hold minimum voltage V_(o) in volts in abscissa. This has resultedin the disadvantage that the resulting apparatus should be made larger.

From the foregoing it will readily be understood that the proportion ofhydrogen to helium and gap length d between the electrodes 1 and 2 aredesirably located in the dotted closed areas shown in FIGS. 30, 31 and32. More specifically, the proportion of hydrogen is not less than 2.5%and the gap length d is not less than 0.5 millimeter while the voltageV_(o) has values of 210, 230 and 240 volts dependent upon the pressureof the filling gas.

While the present invention has been described in conjunction with an ACsource having a voltage of 200 volts it is to be understood that it isequally applicable to AC sources having the voltage higher than that of200 volts, for example, the voltage of 400 volts. In the latter case,the glow current may be low by using a helium-hydrogen mixture includingnot less than 50% by volume of hydrogen which is effective forincreasing the glow hold minimum voltage V_(o) shown at any of thepoints, S, T, U and W illustrated in FIG. 27. This provides a stableglow discharge while being able to decrease the surface area of theelectrodes 1 and 2. In addition, wiring leads may be fine. Therefore theresulting apparatus can be made compact.

Examples of the electrode material may involve copper, aluminum, nickel,pure ion, molybdenum, stainless steel, Kovar (Trade mark) etc. used withvacuum tubes or voltage regulator tubes. However, copper is not suitablefor use in the present invention because the copper has a high currentdensity for the glow discharge to enhance the sputtering therebyseverely deteriorating the insulation of associated insulators. Alsoaluminum is not suitable for used in the present invention because aglow discharge involved transits to an arc discharge with a current aslow as one ampere. Therefore suitable examples of the electrode materialinvolve nickel, pure iron, molybdenum, stainless steel and Kovar (Trademark). The electrode used with the present invention has been formed ofsheet nickel or stainless steel one millimeter thick.

From the foregoing it is seen that the filling of the discharge space 8with a mixture including at least helium and hydrogen can eliminate thetransit of the glow to an arc discharge and the sputtering with a highdischarge current. This gives the result that a stable glow dischargecan be sustained. The reason for which the glow discharge can beprevented from transiting to an arc discharge is to remove oxides on thesurface of the electrodes by the hydrogen included in the fillinggaseous mixture.

The use of the helium-hydrogen mixture is also advantageous in that,only by changing the proportion of the hydrogen to the helium, the glowhold minimum voltage can be selected at will to control the dischargeinput to both electrodes involved as desired.

FIG. 34 shows still another modification of the present invention. Thearrangement illustrated is different from that shown in FIG. 24 only inthat in FIG. 34 the opposite surfaces of the electrodes 1 and 2 arecorrugated to increase the surface areas of the electrodes and anauxiliary electrode 46 is operatively associated with the gap 8 formedbetween the electrodes 1 and 2 as will be subsequently described.

In glow discharge heating apparatus having the discharge input of 5kilowatts, for example, the diameter M of the electrodes 1 and 2 isrequired to be of at least 80 millimeters and also that of theinsulating enclosure 9 is necessarily of at least 100 millimeters. Inother words, the larger the diameter of the electrodes the larger theenclosure 9 and therefore the seal fittings 10 and 11 will be larger.This is attended with the disadvantages that the components becomeexcessively expensive and also a manufacturing cost is increased.

In addition, the opposite surfaces of the electrodes 1 and 2 can beforced toward each other to be crowned in response to a differencebetween a pressure within discharge space 81 and a pressure of a heatedliquid within each electrode so that the bending of the electrodesincreases proportional to the fourth power of the radium M/2 thereof.Accordingly, an increase in diameter of the electrodes may causes theelectrodes 1 and 2 to contact and short circuit each other due to thecrowning thereof.

To avoid this objection, the opposite surfaces of the electrodes 1 and 2have a diametric section of corrugated shape to increase areas of theopposite electrode surfaces with the diameter of the electrodesremaining unchanged. In the arrangement of FIG. 34 each electrode 1 or 2has the diameter M of 52 millimeters and the area of 80 squarecentimeters of that surface thereof opposite to the other electrode 2 or1.

As shown in FIG. 34, the auxiliary electrode 46 is extended and sealedthrough the insulating enclosure 9 so as to center the gap 8 formedbetween the opposite corrugated surfaces of the electrodes 1 and 2 andto be substantially contacted at the free end by the adjacent portion ofthe edge of the gap 8.

Then the AC source 31 is connected at one end to the electrode terminal5 through a normally open switch 45 and at the other end directly to theelectrode terminal 6. The auxiliary electrode 46 is connected to theelectrode terminals 6 and 5 through respective resistors 47 and 48 andalso by a resistor 49 to one output of an auxiliary source circuit 50.The auxiliary source circuit 50 includes the other output connected tothe electrode terminal 5 and therefore the switch 45 and is alsoconnected to the switch 45 through another normally open switch 51 andto the other end of the AC source 31. The operation of theabovementioned circuit configuration will be described hereinafter.

With the auxiliary electrode 46 operatively associated with thedischarge gap 8 as in the arrangement of FIG. 34, the electrodes 1 and 2are called hereinafter the "main electrodes" to be distinguished fromthe auxiliary electrode 46.

In the arrangement of FIG. 34 a glow discharge is fired between the mainelectrodes 1 and 2 after which the glow discharge is smoothly spread onthe corrugated surfaces 1a and 1b respectively of the main electrodes 1and 2. Under these circumstances, a high current can enter the oppositecorrugated surfaces of the main electrodes 1 and 2 as compared withpairs of discharge electrodes including the previously illustratedopposite flat surfaces. Therefore, the discharge input to the electrodescan be increased while the voltage across the main electrodes remainsunchanged.

As a result, the corrugated surface of the main electrodes permits adecrease in diameter thereof attended with a reduction in diameter ofeach of the insulating enclosure 9 and the seal fittings 10 and 11.Accordingly a manufacturing cost can be decreased. Also the corrugatedsurface of the main electrode is effective for preventing main electrodecontact and the crowning of the opposite surfaces thereof.

The opposite surface 1a of the main electrode 1 shown in FIG. 35includes a plurality of grooves of rectangular cross sectionconcentrically disposed at substantially equal intervals thereon.

FIG. 36 shows a plurality of parallel grooves disposed at predeterminedintervals on the discharge surface 1a of the main electrode 1.

The discharge surface 1a of the main electrode 1 shown in FIG. 37includes a plurality of cylindrical depressions disposed in apredetermined pattern thereon.

In the arrangement shown in FIG. 38, a pair of flow confining blocksgenerally designated by the reference numeral 200 and 210 respectivelyare of the same construction and disposed in place within the mainelectrodes 2 and 1 to form heating spaces or flow paths 2A and 1A for aheated liquid therein respectively. The flow confining block 200 isformed of an electrically insulating material such as a syntheticresinous material and includes a feed water tube 201 and a drain tube202 formed in parallel relationship on the exposed end surface thereofto be integral therewith and through openings 201a and 202a connected tothe tubes 201 and 202 respectively. Then openings 201a and 202a open onthat end surface thereof facing the inside of the gap forming surface ofthe main electrode 2 and a peripheral surface thereof respectively. Thetube 201 and the opening 20a interconnected serves as a feed water tubeopening in the flow path 2A while the tube 202 and the opening 202ainterconnected serves as a drain tube also opening in the flow path 2A.

The flow confining block 210 includes a feed water and a drain tubeidentical to those as above described in conjunction with the flowconfining block 200 and designated by like reference numeral identifyingthe corresponding components of the confining block 200 and added withthe numeral 10. For example, the reference numeral 211 designates a feedwater tube.

The flow confining blocks 200 and 210 have the exposed end portionsscrew threaded through the blind cover plate 22 and 23 fixed to the openend portions of the main electrodes 2 and 1 to be flush with the openends thereof respectively.

In other respects, the arrangement is substantially identical to thatshown in FIG. 34 except for the omission of the insulating tubes 37, 38,39 and 40 shown in FIG. 34.

In the arrangement of FIG. 38, the flow confining blocks 200 and 210 canbe removed from the blind cover plates 22 and 23 respectively for thepurpose of inspecting or cleaning the internal surfaces of the mainelectrodes 2 and 1. Therefore the heating efficiency can be alwaysmaintained high.

FIG. 39 shows modification of the arrangement shown in FIG. 15 whereinthe user has access to the heat transfer surfaces of the main electrodesas in the arrangement of FIG. 38 and an auxiliary electrode 46 isoperatively associated with the discharge gap 8. As shown in FIG. 39 aflow confining tube 200 in the form of a hollow cylinder having bothends open is coaxially disposed within the main electrode 1 to form aflow path for a heated liquid therebetween. The cylindrical tube 200 isscrew threaded through a screw member 200a rigidly fitted into the openend of the main electrode 1.

Similarly another flow confining tube 210 in the form of a hollowcylinder having one end closed is detachably connected to the mainelectrode 2 at the outwardly folded end through a screw member 210aformed internally with the tube 210 to form an annular flow path for theheated liquid therebetween.

The flow confining tubes 200 and 210 are of an electrically insulatingmaterial such as a synthetic resinous material.

As in the arrangement of FIG. 38, the flow confining blocks 200 and 210can readily be removed from the main electrodes 1 and 2 respectively forpurposes of inspection and cleaning.

In other respects, the arrangement is substantially similar to thatshown in FIG. 15 except that electric shock preventing means such asabove described in conjunction with FIG. 24 are provided on the feedwater and drain tubes 41, 42 and 43, 44 and the auxiliary electrode 46is operatively coupled to the gap 8 formed between the main oppositeelectrodes 1 and 2.

FIG. 40 shows a different modification of the present invention whichenables decrease of the dimension of the electrically insulatingenclosure and increase of the diameter of the main electrodes. In thearrangement illustrated a pair of main electrodes 1 and 2 identical toeach other are horizontally disposed in opposite relationship to form adischarge gap 8 therebetween. Each of the main electrodes 1 or 2 is inthe form of a hollow cylinder having one end closed and the other endportion 1B or 2B reduced in diameter. The closed flat ends of both mainelectrodes 1 and 2 form therebetween the gap 8 having a width or a gaplength of d and a diameter of M.

Each electrode 1 or 2 includes a shoulder connected to an electricallyinsulating enclosure 9a or 9b in the form of a narrow annulus through afirst annular seal fitting 10a or 11a. Thus the enclosure 9a or 9bencircles the reduced diameter end portion 1B or 2B of the mainelectrode 1 or 2. Then a cylindrical metallic shell 9c or 9d encirclesin spaced relationship the adjacent main electrode 1 or 2 and includes aradially inward directed flange connected at one end to the enclosure 9aor 9b through a second annular seal fitting 10b or 11b. Both shells 9cand 9d have the other ends abutting and fixed together as by welding.Thus the shells 9c and 9d and the main electrodes 1 and 2 formtherebetween an annular discharge space 81 including the gap 8 with theenclosures 9a and 9b, the seal fittings 10a, 10b, 11a and 11b.

The blind cover plate 22 or 23 is rigidly fitted into the open end ofthe main electrode 1 or 2. A feed water tube 41 or 42 is extended andsealed through the blind cover plate 22 or 23 and has an outlet openingsubstantially flush with the internal surface of the cover plate 22 or23. Also a drain tube 43 or 44 is extended and sealed through the blindcover plate 22 or 23 and has an end portion bent into an L in order tofill a heating space 1A or 2A formed of the interior of the mainelectrode 1 or 2 with a liquid to be heated. The end of the L-shape tube43 or 44 faces the uppermost portion of the internal surface of the mainelectrode 1 or 2 with a distance l_(o) maintained therebetween.

Further, the auxiliary electrode 46 and an associated electric circuitare provided in the same manner as above described in conjunction withFIG. 34.

The main electrodes 1 and 2 may be of any desired shape other than thecylindrical shape as above described.

As the main electrodes 1 and 2 are of the same structure, the operationwill now be described in conjunction of one of the electrodes, forexample, the electrode 1.

A liquid to be heated enters the heating space 1A through the feed watertube 42 as shown at the arrow A in FIG. 40 until its liquid surfacereaches a level at which the drain tube 44 opens while the liquid isheated by the main electrode 1. Thereafter the heated liquid isexhausted from the space 1A through the drain tube 44 as shown at thearrow B in FIG. 40. The outflow of the liquid causes a pressure lossacross the drain tube 44 permitting the heated liquid charged in theheating space 1A to have a pressure higher than the atmosphericpressure. In keeping with this increase in pressure, the surface of theliquid within the heating space 1A is forced to be gradually raisedbeyond the open end of the drain tube 44 resulting in a decrease involume of a cavity existing in the heating space 1A.

In this case, the smaller the diameter of the drain tube 44, the greaterthe speed of the liquid flowing through the drain tube 44. As a result,the open end of the drain tube 44 has a lower pressure than the cavitywithin the heating space 1A. This causes an increase in the rate atwhich the drain tube 44 sucks up air left within the heating space 1A.

It has been experimentally proved that the distance l_(o) exceeding 10millimeters causes the air phase in the heating space 1A to be too farspaced from that portion of the liquid just flowing through the open endof the drain tube 44. Therefore the heating space 1A has been difficultto be sufficiently deaerated. This means that the distance l_(o) ispreferably of at most 10 millimeters.

In other words, the distance l_(o) is so dimensioned that, even thoughsteam bubbles are evolved from the liquid being heated within either ofthe heating spaces 1A and 2A and reach the uppermost portion of theheating, spaces they can be rapidly exhausted through the drain tube 43or 44.

After the air has been fully removed from either of the heating spaces1A and 2A as above described, both spaces are entirely filled with theheated liquid without the steam bubbles accumulated to form a cavitytherein. Otherwise a cavity not filled with the heated liquid is formedwithin either of the main electrodes 1 and 2 and therefore that portionthereof contacted by and located adjacent to the cavity excessivelyrises in temperature resulting in its failure.

The arrangement of FIG. 40 is further advantageous in that theinsulating enclosures decrease in diameter and therefore are easilymanufactured with low cost and mechanically strong because theenclosures surround the reduced diameter portions of the main electrodeswhich are encircled by the metallic shells interconnected into a unitarystructure to permit a region occupied by the insulating enclosures to beextremely decreased. Further the main electrodes are insulated from theshells through the insulating enclosures respectively. Accordingly, theresulting apparatus is easy to be manufactured, inexpensive and robustwhile having a long useful life.

In the arrangement shown in FIG. 41, the insulating enclosure 9 in theform of a hollow cylinder having both ends open includes a pair of upperand lower apertured cover plates 13 and 14 respectively connected toboth open ends thereof through annular seal fittings 10 and 11respectively. A pair of hollow main electrodes 1 and 2 having one endopen are vertically disposed in opposite parallel relationship withinthe enclosure 9 to be staggered longitudinally of the enclosure and forma discharge gap 8 in a discharge space 81 defined by the enclosure 9,the seal fittings 10, 11 and the cover plates 13 and 14. The mainelectrodes 1 and 2 have the other open ends fixedly fitted intoapertures on the upper and lower cover plates 13 and 14 to be flush withthe outer surfaces thereof respectively.

The main electrodes 1 and 2 have the open ends closed with blind coverplates 23 and 22 having central openings respectively. Then a L-shapedtube 44 or 41 has one leg connected to the central opening on the blindcover plate 23 or 22 and the other leg horizontally extended to form anoutflow or an inflow tube.

A feed water tube 42 extends in sealing relationship through the one legof the outflow tube 44 and into a heating space 1A within the mainelectrode 1 from above the upper plate 13. Similarly, the drain tube 43extends through the inflow tube 14 and into a heating space 2A withinthe main electrode 2 from below the lower plate 14.

As in the arrangement of FIG. 40, the drain tube 43 has its open endfacing the inside of the closed end of the main electrode 2 through aspacing l_(o) not greater than 10 millimeters.

As shown in FIG. 41, the inflow tube 41 has the end opening in theheating space 2A below the inlet of the drain tube 43 while the feedliquid tube 42 has the end opening in the heating space 1A below theinlet of the drain tube 44. Therefore the heating spaces 1A and 2A canbe entirely filled with the heated liquid as in the arrangement of FIG.40.

Further an auxiliary electrode 46 is operatively associated with thedischarge gap 8 formed between the main opposite electrodes 1 and 2. Ifdesired, both main electrodes may be concentrically disposed.

In the arrangement shown in FIG. 42, a seamless metallic tube is closelywound into a helix 41a or 42a having the outside diameter substantiallyequal to the inside diameter of the main electrode 2 or 1. The helix 41aor 42a includes one end portion 43 or 42 extending through the centralhollow portion thereof and the other end portion 41 or 44 bent into anL-shape. Both helices 41a and 42a are inserted into the main electrodes2 and 1 to be brazed or welded to the internal surfaces thereofrespectively for the purpose of improving the heat transfer from themating main electrodes thereto. A liquid to be heated enters the helix41a or 42a through the end portion 41 or 42 and leaves the end portion43 or 44.

In other respects, the arrangement is identical to that shown in FIG.41.

Each of the main electrodes 1 or 2 can be prevented from corrodingstarting with those portions thereof brazed or welded to the helix 42aor 41a because the brazed or welded portions are not directly contactedby the heated liquid flowing through the helix. Since the heated liquidflows at a high speed through the helix 41a or 42a, the nuclearebullition can be prevented and also a pressure loss in the helix isincreased to prevent steam bubbles from staying in the helix. Thisresults in smooth heat transfer from the main electrode to the heatedliquid flowing through the mating helix. Thus the main electrodes areprevented from excessively rising in surface temperature thereby tosustain stably the glow discharge.

The arrangement shown in FIG. 43 is substantially similar to thatillustrated in FIG. 40 excepting that, in addition to disposing the mainelectrodes 1 and 2 vertically, they are in the form of square hollowprisms and a tube is closely wound in helix complementary in shape tothe interior of the associated main electrode and fixed thereto.

Each of the arrangements shown in FIGS. 42 and 43 is characterized inthat tube means formed of a good thermally conductive material contactsthe internal surface of the mating main electrode to be thermallyintegral therewith and the heated liquid flows through the tube means.This results in the alleviation of limitations as to the configurationof the main electrode while facilitating the manufacturing of theapparatus and prolonging its useful life.

In the arrangement shown in FIG. 44 either of the blind cover plates 22and 23 is provided on that portion diametrically opposite to the normaloutlet with an exhaust port that is, in turn, closed with a plug 221 or231 for example through a screw mechanism. Further an auxiliaryelectrode 46 is operatively coupled to the gap formed between the mainopposite electrodes 1 and 2 as above described in conjunction with FIG.34.

In other respects, the arrangement is substantially identical to thatshown in FIG. 24.

The arrangement shown in FIG. 45 includes the U-shaped flow path orheating space 1A or 2A within the main electrode 1 or 2 and a connectingtube 361 or 351 connected to the heating space 1A or 1B on the inletside. Then the connecting tube 361 or 351 is provided with an exhaustport closed with a detachable plug 231 or 221.

In other respects the arrangement is substantially identical to thatshown in FIG. 44.

When each of the arrangements shown in FIGS. 44 and 45 is desired to beout of service for a long time, the plugs 221 and 231 can be removedfrom the associated exhaust ports to drain the liquid out from interiorof the main electrodes for the purpose of preventing the liquid withinthe main electrode from spoiling or freezing. Also the useful life ofthe apparatus can be prolonged.

While the main electrodes have been described as being in the form ofhollow cylinders having the same shape and disposed in oppositerelationship it is to be understood that the main electrode may be ofany other desired shape. For example, the main electrodes may be in theform of hollow cylinders disposed in coaxial relationship. It isessential that, in order to empty the interior of the main electrodes,the exhaust port must be provided on the lower portions thereof.

While some of the abovementioned Figures, for example, FIG. 34illustrate the control circuit for controlling the glow discharges, FIG.46 shows the fundamental circuit configuration of a control circuit forcontrolling any of the arrangements as above described including onewith no auxiliary electrode. In FIG. 46, the arrangement generallydesignated by the reference numeral 100 comprises a pair of first andsecond electrodes 1 and 2 respectively disposed in opposite relationshipto form therebetween a gap having a gap length or a width d and eachincluding an inflow and an outflow tube. Water enters the interior ofeither electrodes 1 and 2 through the inflow tube to be heated andheated water leaves it through the outflow tube.

The source of AC voltage 31 is connected across the electrodes 1 and 2through a bidirectional triode thyristor 60 with the first electrode 1connected to ground. The bidirectional triode thyristor is calledhereinafter a "Triac" (trade mark). The source 30 is also connectedacross a gate circuit 61 through a normally open switch 62. Then thegate circuit is connected across one electrode and a gate electrode ofthe Triac 60. The switch 62 is closed to fire a glow discharge betweenthe electrodes 1 and 2 thereby to heat a liquid, for example, waterflowing through the interior of each electrode.

The operation of the control circuit shown in FIG. 46 will now bedescribed with reference to FIG. 47 wherein there are illustrated avoltage waveform V supplied from the source 31 and having a peak valueE_(m) and a current waveform V of the glow discharge. As shown in FIG.47, the voltage waveform V in the positive half-cycle of the sourcegradually increases from its null point until time point t₁ is reached.At that time voltage reaches a value of a discharge breakdown voltageV_(f) to fire a glow discharge between the electrodes 1 and 2. At thattime point t₁ a glow current I abruptly flows through the electrodes 1and 2. The glow current I corresponds to a voltage drop expressed byV_(f) -V_(o) where V_(o) designates a glow hold minimum voltage and maybe expressed by I=(V_(f) -V_(o))/R where R designates a dischargeresistance corresponding to a slope of a current-to-voltagecharacteristic curve for a glow discharge as above described inconjunction with FIG. 8.

The at time point t₂ the voltage V is equal to the glow hold minimumvoltage V_(o) after which the glow discharge is extinguished because thevoltage is less than the voltage V_(o).

Thereafter the source 31 enters the next succeeding negative half-cycleof the source in which the process as above described is repeated tocause a glow discharge between the electrodes 1 and 2. In thearrangement shown in FIG. 46 the application of the AC voltage causesthe electrodes 1 and 2 to act alternately as a cathode and an anodeelectrode respectively to be heated because the glow discharge heatsthat electrode acting as the cathode as above described.

From the foregoing it will be seen that, the firing of the glowdischarge at time point t₁ causes an instantaneous increase in glowcurrent so that the glow discharge can not spread following thisincrease in glow current. This results in the tendency to locallyconcentrate the glow current on the electrode to transit the glowdischarge to an arc discharge. The arc charge has a fear that it meltsthe electrode which, in turn, reduces the useful life of the heatingapparatus.

Also the glow current is initiated to flow through the electrodes 1 and2 only upon the source voltage across both electrodes reaching thedischarge breakdown voltage V_(f) while V_(f) >V_(o) holds. Therefore itis impossible to utilize a time interval during which the source voltageis not less than the glow hold minimum voltages V_(o) as a conductiontime resulting in a poor efficiency of utilization of the sourcevoltage.

FIG. 48 shows a control circuit for controlling the glow dischargeheating apparatus of the present invention constructed in accordancewith the principles thereof. The arrangement illustrated comprises anauxiliary source circuit 61 connected across the source of AC voltage 31that supplies AC voltage of 200 volts at the commercial frequency. Thecircuit 61 includes a normally open switch 62, a step-up transformer 63having a primary winding connected across the source 31 through theswitch 62 and a secondary winding having one end connected to theelectrode 2 through a current limiting resistor 64 and the other endconnected to the electrode 1 and also to ground.

As in the arrangement of FIG. 46, the source 31 is connected to theelectrode 2 through the Triac 60. The resistor 64 is connected across aprimary winding of an electrically insulating transformer 65 including asecondary winding connected across a pair of AC inputs of a rectifierbridge 66. The rectifier bridge 66 include a pair of DC outputs one ofwhich is connected to the junction of the source 31 and the Triac 60through a resistor 67 and the other of which is connected to theremaining terminal or a gate terminal of the Triac 60.

The step-up transformer 63 is designed and constructed so that thedischarge breakdown voltage V_(f) is applied across the electrodes 1 and2 before a time point where an instantaneous voltage from the source 31reaches the glow discharge minimum voltage V_(o).

The operation of the arrangement shown in FIG. 48 will now be describedwith reference to FIG. 49 similar to FIG. 47. In FIG. 49 wherein likereference characters designates the components corresponding to thoseshown in FIG. 47, the switch 62 is closed at time point A to permit thesource to apply the source voltage across the primary winding of thetransformer 63. At a point B, a secondary or an output voltage from thetransformer 63 reaches the discharge breakdown voltage V_(f) whereuponthe gap between the electrodes 1 and 2 is broken down to start anelectric discharge therebetween. At that time the output voltage dropsto the glow hold minimum voltage V_(o) (see point C, FIG. 49) for a glowdischarge by means of the current limiting resistor 64. This causes acurrent i on the order of 0.1 ampere to flow through the electrodes 1and 2 resulting in a glow discharge occurring across the electrodes 1and 2. That glow discharge is called a "pilot glow discharge".

The current for the pilot glow discharge causes a voltage drop acrossthe current limiting resistor 64 that, in turn, induces a secondaryvoltage across the transformer 65. The secondary voltage from thetransformer 65 is applied to the gate electrode of the Triac 60 afterhaving been fullwave rectified by the rectifier bridge 67 to put theTriac 60 in its conducting state. Therefore the source voltage isapplied across the electrodes 1 and 2. Under these circumstance, if thepilot glow discharge has not occurred across the electrodes 1 and 2 thenthe pilot glow current i does not flow through the electrodes 1 and 2and no voltage is induced across the insulating transformer 65 with theresult that the Triac 60 is maintained non-conducting. This ensures thatthe application of the high AC voltage across the electrodes 1 and 2does not result in the occurrence of an arc discharge therebetweenunless the pilot glow discharge preliminarily occurs across theelectrodes 1 and 2.

When the source voltage is applied across the electrodes 1 and 2 throughthe now conducting Triac 60 and reaches the glow hold minimum voltageV_(o), the principal glow discharge is fired across the electrodes 1 and2. That is, a current I for the principal glow discharge flows throughthe electrodes 1 and 2. That principal glow discharge current I isextinguished after the source voltage V has again reached the glow holdminimum voltage V_(o) at point E or time point t₂ and therefore theprincipal glow discharge is extinguished. However it is noted that atpoint E the voltage V_(o) from the step-up transformer 63 is appliedacross the electrodes 1 and 2 through the resistor 64 with the resultthat the pilot glow discharge is still established.

Then at point F, the output voltage from the step-up transformer 63 alsobecomes less than the voltage V_(o) to the pilot glow discharge ceases.

Then the source 31 enters the next succeeding negative half-cycle inwhich the process as above described is repeated.

The concept of the embodiment of the present invention as shown in FIG.48 is to apply preliminarily a high voltage across the electrodes bymeans of the auxiliary source circuit to cause the preliminary or pilotglow discharge thereacross and to smoothly derive the principal glowdischarge from the pilot glow discharge. Therefore the arrangement ofFIG. 48 is effective for preventing the principal glow discharge currentfrom abruptly increasing resulting in an arc discharge as in thearrangement of FIG. 46. Further the efficiency of utilization of thesource is increased.

The arrangement shown in FIG. 50 comprises a reactor 68 connectedbetween the source 31 and the electrode 2, and an AC pulse generator 69connected across the source 31 through the normally open switch 62. Thepulse generator 69 includes one output connected by the current limitingresistor 64 to the junction of the reactor 68 and the electrode 2 andthe other output connected to the electrode 1 and therefore to ground.

The gap formed between the electrodes 1 and 2 is so dimensioned that thepeak voltage E_(m) from the source 31 is prevented from effecting thedischarge breakdown of the gap.

As shown in FIG. 51 wherein a voltage and a current waveform V and Irespectively and a pulse waveform P are illustrated, the AC pulsegenerator 69 generates an AC pulse voltage P sufficient to reach thedischarge breakdown voltage V_(f) at time point t₁ where the voltagefrom the source 31 approximately reaches the glow hold minimum voltageV_(o). The pulse voltage P first effects the discharge breakdown of thegap between the electrodes 1 and 2 followed by a flow of the principalglow current I through the electrodes.

As in the arrangement of FIG. 46, the current I becomes null at timepoint t₂ to extinguish the glow discharge after which the process asabove described is repeated in the next succeeding negative half-cycle.

It is noted that the reactor 68 is designed and constructed so that itpresents a high impedance to the pulse waveform P but a low impedance tothe commercial frequency of the source 31.

Thus the arrangement of FIG. 50 ensures that, when the source voltage Vis close to the glow hold minimum voltage V_(o), the principal glowdischarge is initiated between the electrodes 1 and 2 and then theprincipal glow current I is smoothly increased without the transit to anarc discharge.

FIG. 52 shows a modification of the present invention wherein the pilotglow discharge occurs between the auxiliary electrode and either of themain electrodes prior to the principal glow discharge as abovedescribed, for example, in conjunction with FIG. 34. In FIG. 52, themain and auxiliary electrodes 1, 2 and 46 respectively are schematicallyshown and may have any of their structures shown in FIG. 34 and FIGS. 38through 45.

The arrangement illustrated comprises the AC source 31 and an auxiliarysource shown as comprising a step-up transformer 70 including a primarywinding connected across the source 31 through the normally open switch51 and a center-tapped secondary winding. The dot convention is used toidentify the polarity of the instantaneous voltage across the associatedwinding. The secondary winding includes a center tap connected to theauxiliary electrode 46 through a current limiting resistor 71 and anormally open switch 72, and a pair of end terminals connected to themain electrodes 1 and 2 through individual semiconductor rectifierdiodes 73 and 74 with anode electrodes thereof connected to the mainelectrodes respectively. The gap formed between the electrodes 1 and 2has a distance or a gap length d satisfying V_(f) >E_(m) >V_(o), whereV_(f), E_(m) and V_(o) have been previously defined.

The switch 51 is closed to apply the AC voltage V from the AC source 31across the electrodes 1 and 2 while the switch 72 is closed to apply ahigh voltage waveform from the step-up transformer 70 to the auxiliaryelectrode 46. Under these circumstances, when a potential at the mainelectrode 1 is higher than that at the main electrode 2, the diodes 73and 74 are turned off and on respectively to cause a pilot glowdischarge between the auxiliary electrode 46 acting as an anode and themain electrode 2 acting as a cathode. On the contrary, when the mainelectrode 2 is higher in potential than the main electrode 1, the diodes73 and 74 are turned on and off respectively to cause a pilot glowdischarge between the auxiliary electrode 46 acting as an anode and themain electrode 1 as the cathode.

In addition, as the auxiliary electrode 46 has applied thereto thevoltage from the center tap on the secondary transformer 70 winding, thevoltage applied across the auxiliary electrode 46 and the main electrode1 to cause the pilot glow discharge therebetween is quite identical tothat applied across the auxiliary electrode 46 and main electrode 2 tocause the pilot discharge therebetween. Therefore, the transit of thepilot glow discharge due to the auxiliary electrode to the principalglow discharge between the main electrodes 1 and 2 are equally effectedbetween each of the positive half-cycle and the negative half-cycle ofthe source 31.

Further the occurrence of the pilot glow discharge completes a closedcircuit including the diode 73 or 74, the associated half of thesecondary transformer 70 winding, the resistor 71, the closed switch 72and the pilot glow discharge between the auxiliary electrode 46 and themain electrode 1 or 2. This prevents the current for the pilot glowdischarge from entering a circuit with the source 31.

The opening of the switch 72 ceases the pilot glow discharge fromoccurring between the auxiliary electrode 46 and either of the mainelectrodes 1 and 2. Thus the principal glow discharges are not fired inthe next succeeding cycle of the source and the cycles following thelatter with the result that the heating operation is not performed. Inother words, the ON-OFF control of the principal glow discharge can beconducted by turning the pilot discharge on and off.

It is noted that the pilot glow discharge always occurs between theauxiliary electrode 46 acting as the anode and either of the mainelectrodes 1 and 2 acting as the cathode so that the auxiliary electrode46 is not heated. This results in the elimination of the necessity ofcooling the auxiliary electrode.

From the foregoing it is seen that the arrangement, when effecting theON-OFF control of the heating apparatus proper of FIG. 52 ensures thetransit of the glow discharge by turning the pilot glow discharge on andoff.

The arrangement illustrated in FIG. 53 is different from that shown inFIG. 52 only in that in FIG. 53 a zero-voltage firing circuit isprovided to prevent the glow current from abruptly increasing. In FIG.53 a pair of serially connected resistors 75 and 76 are connected acrossthe AC source 31 through the normally open switch 51 to form a voltagedivider, and the junction A of both resistors is connected to a resistor77 subsequently connected to a base resistor 78 that is connected to abase source V_(BB). The resistor 76 is connected to ground. The junctionB of the resistors 77 and 78 is connected to a base electrode of an NPNtransistor 79 including an emitter electrode connected to the resistor76 and a collector electrode connected to a DC source V_(cc) through acollector resistor 80. The transistor 79 has connected across theemitter and base electrodes a semiconductor diode 81 serving to preventa high reverse voltage from being applied across those electrodes andalso connected across the collector and emitter electrodes adifferentiating circuit including a capacitor 82 and a resistor 83. Thejunction of that collector electrode and the capacitor is designated bythe reference character C and the junction of the capacitor 82 and theresistor 78 is designated by the reference character D only for purposesof illustration.

The junction D is connected to one AC input of a rectifier bridge 84including the other AC input connected to the resistor 83. The rectifierbridge 84 includes a pair of DC outputs connected across a resistor 85that is connected at one end to a gate electrode of a Triac 87 through anormally open switch 86 and at the other end to the primary winding ofthe transformer 70. The Triac 87 is connected across AC source throughthe primary transformer 70 winding and the switch 51 and has connectedthereacross a series combination of a capacitor 88 and a resistor 89serving as an absorber.

The components 75 through 89 as above described form a zero-voltagefiring circuit generally designated by the reference numeral 90.

With the switch 51 closed, an AC voltage developed at the point A issimilar to the source voltage and sinusoidal as shown at waveform A inFIG. 54. The AC sinusoidal voltage passes through its zero voltagepoints at time points t_(o), t₁ and t₂ in each cycle of the source 31.Assuming that the source V_(BB) is at a null potential, a voltagedeveloped at the point B is sinusoidal between time points t_(o) and t₁or in the positive half-cycle of the source and remains null betweentime point t₁ and t₂ or in the negative half-cycle thereof by means ofthe action of the diode 81 as shown at waveform B in FIG. 54. Since thetransistor 79 is turned on only in response to a voltage applied to thebase electrode to render the latter positive with respect to the emitterelectrode, the same is in its ON state between time points t_(o) and t₁and in its OFF state between time points t₁ and t₂. Accordingly, avoltage developed at the point C is null when the transistor 79 is inits ON state and equal to a voltage across the source V_(CC) alsodesignated by V_(CC) when it is in its OFF state as shown at waveform Cin FIG. 54.

The voltage at the point C is differentiated by the differentiatingcircuit 82, 83 to produce alternately a negative and a positive pulse atthe point D as shown at waveform D in FIG. 54. Those pulse are rectifiedby the rectifier bridge 84 to form positive pulses which appear at apoint E connected to the switch 84 at time points t_(o), t₁ and t₂ asshown at waveform E on FIG. 54.

With the switch 86 closed, the pulses shown at waveform E in FIG. 54 aresuccessively applied to the gate electrode of the Triac 87. In otherwords, gate pulses are necessarily developed at the gate electrode ofthe Triac 87 at the zero passage points of the source voltage or at timepoints t_(o), t₁ and t₂. Thus it is seen that, even though the switch 86has been closed at any time point, the Triac 87 is brought into its ONstate starting with the zero passage point of the source voltage. As aresult, a pilot voltage from the transformer 70 is applied to theauxiliary electrode 46 starting with the zero passage point of thesource voltage or time point t_(o), t₁ or t₂ with the result that theprincipal glow current is prevented from sharply increasing. This meansthat a liquid flowing in heat transfer relationship along the internalsurface of each electrode 1 or 2 is smoothly heated.

The arrangement of FIG. 53 is advantageous in that a principal glowcurrent is prevented from sharply rising at a firing time point and theglow discharge is prevented from transiting to an arc discharge due tothe local concentration of the current while efficiency of utilizationof the source voltage is high.

If desired, the zero voltage firing circuit 90 may be formed of solidstate relays.

In the arrangements shown in FIGS. 52 and 53 the auxiliary sourcecircuit including the step-up transformer is formed of components havingstray capacitances between one another and with respect to ground withthe switch 72 put in its open position. This results in a fear that apotential at the auxiliary electrodes 46 would be raised due to thosestray capacitances until a voltage across the auxiliary electrode 46 andeither of the main electrodes 1 and 2 exceeds the discharge breakdownvoltage across the associated gap. This results in the undesirableoccurrence of a glow discharge between the main electrodes 1 and 2 whichprevents the principal glow discharge from being controlled with thepilot glow discharge.

In order to avoid this objection, the arrangement illustrated in FIG. 55includes a pair of dummy resistors 93 and 94 connected between the diode73 and the resistor 71 and between the diode 74 and the resistor 71respectively. The resistors 93 and 94 are effective for determining thepotential at the auxiliary electrode 46 so as to prevent the voltageacross the auxiliary electrodes 46 and either of the main electrodes 1and 2 from exceeding the discharge breakdown voltage across the gap asabove described.

In other respects, the arrangement is identical to that shown in FIG. 53except for the omission of the switch 72.

The auxiliary electrode 46 is normally positioned to be equidistant fromboth main electrodes 1 and 2 and therefore the resistors 93 and 94 areequal in resistance value to each other in order to set the voltageacross the auxiliary electrode 46 and the main electrode 1 equal to thatacross the electrodes 46 and 2 with the switch 62 put in its openposition. Even under these circumstances, it is to be understood thatthe gap length between the auxiliary electrode 46 and either of the mainelectrodes 1 and 2, and the type and pressure of a dischargeable gasshould be preliminarily determined so as to prevent the occurrence of adischarge breakdown between the auxiliary electrode 46 and either of themain electrodes 1 and 2 with the switch 62 put in its open position.

The arrangement illustrated in FIG. 56 is different from that shown inFIG. 55 only in that in FIG. 56 a Triac is substituted for the switch 62in order to permit the ON-OFF operation to be repeatedly performed witha high frequency. As shown in FIG. 56, a Triac or a bidirectional triodethyristor 95 is located in place of the switch 62 shown in FIG. 55. TheTriac 95 includes a gate circuit 95 connected to a gate electrodethereof to deliver trigger signals to the gate electrode to turn theTriac 95 on and off and a series combination of a capacitor 97 and aresistor 98 serving as an absorber.

If desired, the Triac 95 may be included in the zero voltage firingcircuit 90.

When the pilot glow discharge has the discharge breakdown characteristicwith a fairly long time delay, the pilot glow discharge may be fired ata time point where the source voltage approaches its peak value providedthat the Triac 95 has flowing therethrough a current in excess of itsholding current. This is attended with the occurrence of the principalglow discharge having a sharply rising current. A current for this glowdischarge may sharply rise. In this case, a negative glow included inthe principal discharge can not spread following an increase in currentto locally concentrate the current resulting in a danger that the glowdischarge transits to an arc discharge. In order to avoid this danger,it is necessary to determine magnitudes of resistances 93 and 94 and animpedance on the primary side of the step-up transformer 70 high enoughto prevent a flow of current through the Triac 95 in excess of itsholding current.

In the arrangement illustrated in FIG. 57 an electronic switch 98 suchas a thyristor with a trigger circuit 99 is connected between theresistor 71 and the junction of dummy resistors 93 and 44 as shown inFIG. 57. When a voltage drop across the serially connected resistors 93and 94 decreases to some extent, and when the electronic switch 98 isput in its ON state by the trigger circuit 99, a current flowing throughthe electronic switch 98 may exceed its holding current even in theabsence of a pilot glow discharge. Under these circumstances, if thepilot glow discharge has the discharge breakdown characteristic with along time delay, there is a danger that the resulting glow dischargetransits to an arc discharge as above described. In order to avoid thisdanger, the resistors 93 and 94 are required to be somewhat high inresistance.

Alternatively the electronic switch 98 with its trigger circuit 99 maybe connected between the junction of the dummy resistors 93 and 94 andthe auxiliary electrode 46 as shown in FIG. 58. In this case, theresistors 93 and 94 are not particularly subjected to limitations as totheir resistances unless a voltage across the auxiliary electrode 46 andeither of the main electrodes 1 and 2 is reduced.

The arrangements shown in FIGS. 55 through 58 ensure that the principalglow discharge is controlled with the pilot glow discharge. This isbecause the dummy resistors prevent the potential at the auxiliaryelectrode from floating due to stray capacitances as above described inconjunction with FIGS. 52 and 53 and the like in the absence of thevoltage applied to the auxiliary electrode.

The arrangement illustrated in FIG. 59 comprises an electricallyisolating transformer 141 including a primary winding connected acrossthe AC source 31 and a secondary winding connected across a seriescombination of a rectifying diode 142, a current limiting resistor 143and capacitor 144, and an NPN transistor 145 including an emitterelectrode connected to one side of the capacitor 144 and a collectorelectrode connected to the other side of the capacitor 144 through asemiconductor diode 146 for absorbing back pulses. The transistor 145includes a base electrode connected to a gate circuit 149 which is alsoconnected to the emitter electrode thereof to turn the transistor 145 onand off.

The components 141 through 146 form a high voltage pulse generatorcircuit generally designated by the reference numeral 140 with a step-uppulse transformer 147 which includes a primary winding connected acrossthe diode 146 and a secondary winding connected to a semiconductor diode148 for shaping a pulse waveform.

As in the arrangement of FIG. 57, the diode 148 is connected to theresistor 71 which is connected to the auxiliary electrode 46 through thethyristor 98 which is turned on and off by a trigger circuit 99.Further, the serially connected dummy resistors 93 and 94 are connectedacross the main electrodes 1 and 2 and also, through the switch 51,across the AC source with the junction of both resistors connected tothe auxiliary electrode 46.

The operation of the arrangement shown in FIG. 59 will now be describedwith reference to FIG. 60 wherein there are illustrated a voltagewaveform V across the main electrodes 1 and 2 and a no-load voltagewaveform V_(N) at the auxiliary electrode 46. With the main electrode 1disposed opposite to the main electrode 2 to form therebetween apredetermined gap fulfilling the relationship that the dischargebreakdown voltage V_(f) for the gap is higher than the peak value E_(m)of the source voltage under the predetermined discharge conditions, theswitch 51 is closed to apply the AC voltage across both electrodes 1 andfrom the source 31. Also, the source 31 charges the capacitor 144 withthe polarity illustrated through the transformer 141, the diode 142 andthe resistor 143. Then gate and trigger circuits 149 and 99 respectivelyapply simultaneously respective gate signals to the transistor 145 andthe thyristor 99 to turn them on. The turn-on of the transistor 145causes the charged capacitor 144 to discharge through the primarywinding of the pulse transformer 147 and the now conducting transistor145. As a result, a pulse voltage stepped up by the pulse transformer147 is supplied from the secondary winding thereof through the diode148, the resistor 71 and the now conducting thyristor 98 to theauxiliary electrode 46. It is noted that the circuits 149 and 99generate the respective pulses before the voltage across the mainelectrodes 1 and 2 reaches the discharge breakdown voltage V_(o). Asshown in FIG. 60, the circuits 149 and 99 generate the pulses at timepoint t₂ before time point t_(o) where the source voltage reaches thedischarge breakdown voltage V_(o) in each positive half cycle thereofand the pulses terminates shortly after time point t_(o). That is, eachpulse has a predetermined pulse width a little longer than a timeinterval between time points t₂ and t_(o). Each pulse is shown atwaveform V_(N) in FIG. 60 as being superposed on that portion of thesource voltage divided by the resistors 93 and 94, assuming that bothresistors have resistance values equal to each other. In the nextsucceeding negative cycle of the source voltage the pulse is similarlydeveloped at time point t₃ before time point t₁ where the voltage acrossthe main electrodes 1 and 2 reaches the negative value -V_(o) of thedischarge breakdown voltage and terminates shortly after time point t₁to have the same pulse width as that appearing in the positivehalf-cycle of the source voltage.

In the arrangement of FIG. 59 it is required to cause a pilot glowdischarge before time point t_(o) or t₁ by applying the pulse waveformV_(N) to the auxiliary electrode 46 as above described. Also it isrequired to select the pulse width so as to reliably effect thedischarge breakdown of the gap between the auxiliary electrode andeither of the main electrodes 1 and 2 and within the duration of theassociated pulse.

In general, a time delay is caused after the voltage has been appliedacross discharge gaps and until the discharge breakdown is accomplishedtherebetween. It is well known that this time delay is equal to the sumof a time interval between the application of the voltage across adischarge gap and the appearance of a first electron resulting in theinitiation of development of the electron avalanche and another timeinterval between the initiation of development of an electron avalancheand the completion of a stead-state discharge. The first mentional timeinterval is called a statistic delay and the latter is called aformation delay. The statistic delay is overpoweringly long.

Assuming that a voltage applied across the particular discharge gap hasthe peak value higher that a voltage effecting the DC breakdown of thedischarge gap, stepped voltages are applied across the discharge gapn_(o) times. Assuming that, among them the n applications of the voltagehas time delays not shorter than τ and (n+Δn) applications thereof hastime delays not shorter than (τ+Δτ),

    Δn=-AnΔτ

holds where A designates a constant. Thus

    n=n.sub.o e.sup.-Aτ

is fulfilled by the statistic delay. The above expression may be plottedinto a straight line with the axes of ordinates and abscissasrepresenting the values of n and τ respectively on a semilogarithmicscale. A graphic representation thus plotted is called a Laue plot.

FIG. 61 shows on example of the Laue plot. In FIG. 61 an extremity of anauxiliary electrode having a diameter of 3 millimeters is located at anedge of a gap of 3 millimeters formed between a pair of main oppositeelectrodes to form a spacing of about 1 millimeter between the extremityof the auxiliary electrode and either of the main electrodes. The gapwas filled with a discharge gap formed of a mixture including 89% byvolume of helium and 11% by volume of hydrogen under a pressure of 100Torrs. In FIG. 61 the reference numerals 150, 151, 152 and 153 depictthe source voltages having the peak values of 600, 800, 1000 and 1200volts respectively. From a stepped curve 152, for example, it is seenthat for the peak source value of 1000 volts the time interval betweenthe t₂ and t_(o) or between the t₃ and t₁ (see FIG. 60) must be of atleast 250 microseconds. Also the auxiliary source for the pilot glowdischarge should have a current capacity of at least about 10milliamperes in order to transit smoothly from the pilot glow dischargeto the principal glow discharge.

By taking account of a time delay with which the discharge gap is browndown with the pulse voltage of the voltage waveform V_(N) shown in FIG.60, the waveform V_(N) is given a pulse width or a duration defined bythe time intervals ranging from time point t₂ or t₃ to time point t_(o)or t₁ respectively while the current capacity of the auxiliary source isdetermined as required for transiting the pilot glow discharge to theprincipal glow discharge and the pulse voltage delays rapidly at andafter time point t_(o) or t₁. This measure ensures that the pilot glowdischarge is always caused prior to time point t_(o) or t₁ and theprincipal discharge current surely rises at time point t_(o) or t₁.

After the principal glow discharge has been caused between the mainelectrodes 1 and 2, discharge energy from the principal glow dischargeas thermal energy alternately enters the main electrodes 1 and 2 withthe result that a liquid flowing in contact through either of the mainelectrodes is instantaneously heated.

The arrangement of FIG. 59 is advantageous in that the principaldischarge current smoothly rises to cause the development of a negativeglow involved to satisfactorily follow up a change in discharge currentthereby to prevent the local concentration of the current without theglow discharge transiting to an arc discharge while the source voltageis efficiently utilized. This is because the auxiliary electrode isadapted to be applied with a pulse voltage that rises before a timepoint where a voltage applied across the main electrodes reaches a glowhold minimum voltage across the main electrodes thereby to always firethe pilot glow discharge before that time point and rapidly falls to itsnull value at and after said time point. Also the use of the pulsewaveform is effective for decreasing the power capacity of the auxiliarysource and therefore reducing a dimension and a cost thereof.

FIG. 62 shows a modification of the arrangement shown in FIG. 59. Thearrangement illustrated comprises a pair of electrically isolatingtransformers 141 and 155 including a common iron core and a commonprimary winding connected across the AC source 31 through the normallyopen switch 51, the high voltage pulse generator circuit 140 as abovedescribed in conjunction with FIG. 59 connected to the transformer 141,and a current supply circuit generally designated by the referencenumeral 154 and connected across the transformer 155.

The current supply circuit 154 includes a center-tapped secondarywinding of the transformer 155, and a pair of semiconductor diodes 156and 157. The diode 156 is connected at the anode electrode to one sideof the source 31 through the switch 51 and therefore the main electrode1 while diode 157 is connected at the anode electrode to the other sideof the source 31 and therefore the main electrode 2 that is, in turn,connected to ground. The center tap on the secondary transformer 155winding is connected to the output of the pulse generator circuit 140 orthe junction of the diode 148 and the current limiting resistor 71.

In other respects, the arrangement is identical to that shown in FIG.59. The dot convention is used to identify, the polarity of theinstantaneous voltage developed across the associated transformerwinding.

The current supply circuit 155 is operative to full-wave rectify an ACvoltage induced across the secondary transformer 155 winding and supplya current due to the full-wave rectified voltage to the auxiliaryelectrode 46 through the resistor 71 and the thyristor 98 with the pulsevoltage from the pulse generator circuit 140.

In the arrangement in FIG. 62, the discharge gap between the mainelectrodes 1 and 2 has been dimensioned as above described inconjunction with FIG. 59 and the switch 51 is closed to supply thesource voltage across the main electrodes 1 and 2. The source voltage isa commercial AC voltage having a frequency of 60 hertzs as shown bydashed waveform V in FIG. 63 wherein its cycle has a duration of 16.7milliseconds.

The pulse generator circuit 140 generates a high voltage pulse in eachof the half-cycles of the source voltage in the same manner as abovedescribed in conjunction with FIG. 59. After having been shaped by thediode 148, the high voltage pulse is developed on the resistor 71 andsuperposed on the full-wave rectified voltage from the current supplycircuit 154 also applied to the resistor 71 as shown at voltage waveformV_(N) in FIG. 62. Then pulse voltage V_(N) superposed on the voltagefrom the current supply circuit 154 is supplied to the auxiary electrode46 through the conducting thyristor 98.

From FIG. 63 it is seen that the voltage waveform V_(N) includes thefull-wave rectified component having a voltage relative to the mainelectrode 2 equal to a voltage V_(op) for the pilot glow discharge at atime point t₆ in the positive half-cycle of the source voltage, and alsoa voltage relative to the main electrode 1 equal to that voltage V_(op)at time point t₇ in the negative half-cycle thereof. Time points t₆ andt₇ are ahead of time points t₀ and t₁ respectively where the sourcevoltage is equal to the glow hold minimum voltage V_(O).

With the main electrode 1 higher in potential than the main electrode 2,the diodes 156 is in its OFF state while the diode 157 is in its ONstate tending to cause a pilot glow discharge between the auxiliaryelectrode 46 and the main electrode 2. On the contrary, with the mainelectrode 1 is lower in potential than the main electrode 2, the diodes156 and 157 are turned on and off respectively. This tends to cause apilot glow discharge between the auxiliary electrode 46 and the mainelectrode 1. In each case, the voltage across the auxiliary and mainelectrode 46 and 1 respectively is equal to that across the auxiliaryand main electrode 46 and 2 respectively so that a current for the pilotglow discharge remain unchanged. With the auxiliary electrode 46equidistant from the main electrodes 1 and 2, the transit of the pilotglow discharge to the principal glow discharge between the mainelectrodes 1 and 2 is accomplished in a similar manner in both cases.

The voltage waveform V_(N) also includes a pulse waveform component fromthe pulse generator circuit 140 rising at time point t₂ or t₃ behindtime point t₆ or t₂ and falling at time point t₄ or t₅ ahead of timepoint t₀ or t₁. The pulse waveform component results from a gate pulseP₁ from either of the gate and trigger circuits 149 and 99 rising andfalling simultaneously with the rise and fall of the associated pulsecomponent. The pulse waveform component is required to have a pulsewidth sufficient to effect the discharge breakdown of the gap betweenthe auxiliary electrode 46 and either of the main electrodes 1 and 2. Itis to be noted that it is not required to cause time point t₄ or t₅ tocoincide with time point t₂ or t₁ respectively as in the arrangement ofFIG. 59 and that the pulse width may be sufficiently shorter than thatrequired for the latter. In addition, the discharge breakdown scarcelyrequires a current resulting in the pulse generator circuit 140 beingsubstantially reduced in power capacity.

The gate pulse from each of the gate and trigger circuits 149 and 99should have a rise time fulfilling the following requirements: The gatepulse P₁ should rise at time point t₂ or t₃ required to be behind timet₆ or t₇ respectively while the pilot glow discharge should be causednot later than time point t₀ or t₁. Otherwise the principal dischargecurrent is too sharply raised to cause the spread of the particularnegative glow to follow this rise in current resulting in a danger thatthe current is locally concentrated on either of the main electrode topermit the glow discharge to transit to an arc discharge. Also thesource voltage can be utilized only with a low efficiency. Thus timepoint t₄ or t₅ should be ahead of time point t₀ or t₁ respectively.

With the gate pulse P₁ generated to fulfill the requirements as abovedescribed, the pilot glow discharge is always caused ahead of time t₀ ort₁ in the positive or negative half-cycle of the source voltage and theglow discharge current through the main electrodes 1 and 2 smoothlyrises at and after time point t₀ or t₁ in the positive or negativehalf-cycle of the source voltage. Accordingly, the principal glowdischarge is established resulting in the instantaneous heating of theparticular liquid contacted by either of the main electrodes 1 and 2.

Further it is required to make the peak voltage value of the sinusoidalcomponent of the voltage waveform V_(N) less than the dischargebreakdown voltage for the gap between the auxiliary electrode 46 andeither of the main electrodes 1 and 2 thereby to prevent the pilot glowdischarge from firing with the sinusoidal component. Alternatively, itis required to impart a high value to each resistance 93 or 94 toprevent the voltage waveform V_(N) from being applied to the auxiliaryelectrode 46 in the absence of the gate pulse P₁ and to prevent acurrent flowing through the thyristor 98 via the resistors 93 and 94from exceeding the holding current thereof when the pilot glow dischargeis not fired. Also the diodes 156 and 157 must have such reverse voltagewithstanding characteristic that both diodes are not broken down withthe high voltage pulses generated by the pulse generator circuit 140.

If desired, the pulse generator circuit may utilize a peak transformer.

The arrangement of FIG. 62 is advantageous in that the pulse generatorcircuit can be reduced in power capacity resulting in the provision ofan auxiliary source circuit easy to be manufactured and inexpensive.This is because the pulse generator circuit for effecting the dischargebreakdown of the pilot glow discharge gap is separated from the circuitfor supplying current to the main electrodes after this dischargebreakdown.

FIG. 64 shows a different modification of the present invention drivenby a three-phase AC source. The arrangement illustrated comprises threemain electrodes 1U, 1V and 1W radially disposed by having theirlongitudinal axes arranged at equal angular intervals of 120 degrees.The main electrodes are in the form of hollow cylinders having one endclosed into a crown shape that, in turn, faces the remaining closed endsof the same shape. The main electrode 1U, 1V and 1W include the otherend portions rigidly fitted into respective annular supporting members14U, 14V and 14W interconnected through enclosure portions 9 formed ofan electrically insulating material such as glass porcelain or the likeand seal fittings 10U, 10V and 10W connected to both adjacent supportingmembers and the adjacent edges of the enclosure portions 9 to define ahermetic discharge space. The other end of each electrode 1U, 1V or 1Wis closed with a blind cover plate 23U, 23V or 23W having an inflow tube42U, 42V or 42W and an outflow tube 44U, 44V or 44W extended and sealedtherethrough.

Three auxiliary electrodes 46U, 46V and 46W are radially extended andsealed through the enclosure portions 9 respectively to be equidistantfrom the adjacent main electrodes and each includes an end portion benttoward the associated main electrodes to form very narrow gapstherebetween. For example, the auxiliary electrode 46U is radiallyextended and sealed through the enclosure portion 9 disposed between themain electrodes 1U and 1V and includes the end portion bent toward themain electrode 1U so as to cause a pilot glow discharge. Each of theauxiliary electrodes is coated with the same electrically insulatingmaterial as the enclosure portion 9 except for both the end facing theassociated main electrode and that portion externally protruding fromthe mating enclosure portion 9.

A three-phase source is represented by source terminals U, V and W whichare connected to annular electrode terminals 6U, 6V and 6W fitted ontothose portions of the main electrodes, 1U, 1V and 1W disposed externallyof the enclosure portions 9 respectively. Each of the auxiliaryelectrodes is connected to the electrode terminals disposed on theadjacent main electrodes through individual dummy resistors. Forexample, the auxiliary electrode 46U is connected to the electrodeterminal 6U of the main cylinder 1U through the dummy resistor 47U onthe one hand and to the electrode terminal 6V of the main electrode 1Vthrough the dummy resistor 48U.

The auxiliary electrode 46U is also connected by a current limitingresistor 49U to an auxiliary source circuit 50 also connected to theelectrode terminal 6U. The auxiliary source circuit 50 is furtherconnected across the source terminals U and V through a normally openswitch 51U connected to the source terminal V.

A circuit identical to that above described is provided for each of theremaining main electrodes and the auxiliary electrode operativelyassociated therewith and includes the components identical to thoseabove described. Therefore the identical components are designated bylike reference numerals suffixed with the reference character U, V or Widentifying the mating source terminal or the phase of the three-phasesource.

The operation of the arrangement shown in FIG. 64 will now be describedwith reference to FIG. 65 wherein there are illustrated voltage andcurrent waveforms developed at various points in the arrangement of FIG.64 with a voltage V_(U) applied to the main electrode 1U being selectedas a reference.

While a liquid to be heated is flowing through the interior of each mainelectrode via the associated inflow tube and leaves the mating outflowtube a three phase voltage is applied to the main electrodes 1U, 1V and1W through the source terminals U, V and W and all the switches 51U, 51Vand 51W are put in their closed position. At time point t₁ shortlybefore a voltage (see waveform V_(V), FIG. 65) applied across the mainelectrodes 1U and 1V reaches a glow hold minimum voltage V_(O), a highvoltage pulse (see waveform P_(Uo), FIG. 65) from the auxiliary sourcecircuit 50U is applied to the auxiliary electrode 46U to cause a pilotglow discharge across the narrow gap between the auxiliary electrode andmain electrodes 46U and 1U respectively with the main electrode 1Uacting as a cathode. This pilot glow discharge is caused with a lowcurrent, and upon time point D being reached, it instantaneously inducesa glow discharge between the main electrodes 1U and 1V with theelectrode 1U acting as a cathode. The latter discharge spreads throughthe surface of both main electrodes 1U and 1V and is sustained aftertime point D.

Then when a voltage (see waveform V_(W), FIG. 65) applied across themain electrodes 1U and 1W exceeds the glow hold minimum voltage V_(O) attime point E, the glow discharge developed between the main electrodes1U and 1V plays a role of the pilot glow discharge to cause a glowdischarge between the main electrodes 1U and 1W at and after that timepoint with the main electrode 1U acting as a cathode.

At time point F a voltage across the main electrodes 1V and 1W is equalto the voltage V_(O) but no discharge is caused between those mainelectrodes because of the absence of a pilot glow discharge with themain electrode 1V acting as a cathode. Therefore a high voltage pulse(see waveform P_(Vo), FIG. 65) from the auxiliary source circuit 50V isapplied to the auxiliary electrode 46V at time point t₂ shortly ahead oftime point F to cause a pilot glow discharge between the auxiliary andmain electrodes 46V and 1V respectively. That pilot glow dischargesimilarly causes a glow discharge between the main electrodes 1V and 1Wat and after time point F with the main electrode 1V acting as acathode.

When time point G is reached, the voltage V_(V) across the mainelectrodes 1U and 1V is equal to the voltage V_(O) and the glowdischarge caused between the main electrode 1V acting as the cathode andthe main electrode 1W plays a role of a pilot glow discharge. Thiscauses a glow discharge between the main electrode IV acting as acathode and the main electrode 1W at and after time point G.

Similarly, since the voltage V_(W) across the main electrodes 1W and 1Uexceeds the voltage V_(o) at time point H, a high voltage pulse (seewaveform P_(Wo), FIG. 65) from the auxiliary source circuit 50W has beenpreliminarily applied to the auxiliary electrode 46W at time point t₃shortly ahead of time point H to cause a pilot glow discharge betweenthe auxiliary electrode 46W and the main electrode acting as a cathode.The pilot glow discharge between the auxiliary and main electrode 46Wand 1W respectively transits to a glow discharge caused between the mainelectrode 1W acting as a cathode and the main electrode 1U at and aftertime point H.

Then at time point I the voltage V_(W) across the main electrodes 1V and1W exceeds the voltage V_(O) so that the glow discharge between the mainelectrodes 1W and 1U serves as a pilot glow discharge to cause a glowdischarge between the main electrode 1W acting as a cathode and the mainelectrode 1U until one cycle of the source voltage is completed.

Therafter the process as above described is repeated to cause repeatedlyglow discharge between pairs of the main electrodes. When acting as thecathode, the main electrodes successively heat the liquid therein.

From the foregoing it will readily be understood that the gate pulsesare repeatedly applied to the auxiliary electrodes 46U, 46V and 46W attime points t defined by

    t=t.sub.1 +nT, t=t.sub.2 +nT and T=t.sub.3 +nT

respectively where T designates a period of the three-phase sourcevoltage and n indicate any positive integer including zero.

In FIG. 65 solid current waveform I_(U) designates a glow dischargecurrent with the main electrode 1U acting as the cathode, dotted currentwaveform I_(V) designates the glow discharge current with the mainelectrode 1V acting as the cathode and broken current waveform 1Wdesignates the glow discharge current with the main electrode 1W actingas the cathode. The reference characters P_(Uo), P_(Vo) and P_(Wo)designate no-load pulse waveforms which change to the actual pulsewaveforms P_(U), P_(V) and P_(W) respectively after the associated pilotglow discharges have been fired.

Also it is noted that FIG. 65 illustrates the waveforms developed duringa time interval equal to twice the period T of the source voltage V_(V)applied across the main electrodes 1U and 1V and that the polarity ofthe current waveforms have not been considered.

From the foregoing it will readily be understood that the glow dischargehas a time period equal to three times that provided by a single-phasesystem and therefore the three-phase apparatus triple in power capacityover a single-phase apparatus.

In the arrangement of FIG. 63 the auxiliary electrode is disposedbetween each pair of adjacent main electrodes for the purpose ofcontrolling thermal energy entering each of the main electrodes. Howeverit is included in the scope of the present invention to replace theauxiliary electrode by a bidirectional triode thyristor seriallyconnected to each main electrode to control thermal energy enteringthereinto through the ON-OFF operations of the thyristors.

The arrangement illustrated in FIG. 66 is different from that shown inFIG. 64 only in that in FIG. 66 a combination of a pulse transformer70U, 70V or 70W and a high voltage pulse generator circuit 140U, 140V or140W is substituted for each auxiliary source circuit. The combinationof the pulse transformer and pulse generator circuit may be identical tothe pulse generator circuit 140 shown in FIG. 59.

Also the main and auxiliary electrodes are schematically illustrated inFIG. 66 and may be similar to those shown in FIG. 64 and the resistors48U, 48V and 48W are omitted.

FIG. 67 shows another modification of the arrangement shown in FIG. 66.In the arrangement illustrated, the electrically isolating transformer70 includes a primary winding W₁ connected across the source terminals Uand V through the switches 51 and a pair of secondary windings W₂ and W₃connected respectively across a high voltage pulse generator circuit 140such as above described in conjunction with FIG. 59 and a gate circuit161. The pulse generator circuit 140 includes one output connected tothe source terminal U and the other output connected to anode electrodesof thyristors S_(U), S_(V) and S_(W) through the common current limitingresistor 49. The thyristors S_(U), S_(V) and S_(W) include cathodeelectrodes connected to the auxiliary electrodes 46U, 46V and 46Wrespectively. The gate circuit 161 is connected to the thyristors S_(U),S_(V) and S_(W) to control the firing thereof.

In other respects, the arrangement is substantially identical to thatshown in FIG. 66.

FIGS. 68A-68D illustrate voltage and current waveforms developed atvarious points in the arrangement shown in FIG. 67. From the comparisonof FIG. 68A with FIG. 65 it is seen that voltage and current waveformsshown on the are substantially similar to those illustrated in FIG. 65and pulse waveforms Po are substituted for the pulse waveforms P_(U)-P_(Uo), P_(V) -P_(Vo) and P_(W) -P_(Wo) shown in FIG. 65. Thus likereference characters have been employed to identify the waveformscorresponding to those illustrated in FIG. 65. Thus the arrangement issubstantially identical in operation to that shown in FIG. 66.

As seen in FIG. 68B, the gate circuit 161 applies a gate pulse (seewaveform G_(U)) across the gate and cathode electrodes of the thyristorS_(U) shortly before the high voltage pulse (see waveform Po from thepulse generator 140 is supplied to the auxiliary electrode 46U to bringit in its conducting state and then the pulse Po is supplied to theauxiliary electrode 46U through the resistor 49 and the now conductingthyristor S_(U). This is true in the case of the remaining pulses Ppassing through the respective thyristors S_(V) and S_(W).

Each of the gate pulses shown at waveforms G_(U), G_(V) and G_(W) inFIGS. 68B-68D should have a pulse width sufficient to ensure that apilot glow discharge is fired between the associated auxiliary and mainelectrodes such as shown by 46U and 1U and transits to the principalglow discharge caused between the mating main electrodes such as shownby 1U and 1V. That is, the gate pulse should be at least sustained untila time point is reached where the associated source voltage, forexample, the voltage V_(V) exceeds the glow minimum voltage V_(o). Ifthe pilot glow discharge causes a current flowing through the associatedthyristor to exceed its holding current then the gate pulse may continueuntil the pilot glow discharge is fired.

The arrangement of FIG. 67 is advantageous over that shown in FIG. 66 inthat the resulting circuit is simple, small-sized and inexpensivebecause of the provision of a single high voltage pulse generatorcircuit.

In the preferred embodiments of the present invention, the mainelectrodes and associated components, such as the flow confining tubes,the connecting tubes, the inflow and outflow tube, the blind coverplates shown, for example, in FIG. 24 are formed of metallic materialand put in contact with a heated liquid that is electrolytic. This mayresult in a fear that those metallic components are corroded with theheated liquid and reduced in useful life. Particularly the mainelectrodes and those tubes directly connected thereto have highprobabilities of electrolytic corrosion because the source voltage isdirectly applied across the main electrodes while the inflow and outflowtubes are connected to ground thereby permitting currents to flow in tothe main electrodes and those tubes through the heated electrolyticliquid.

The arrangement shown in FIG. 69 includes corrosion preventingelectrodes for preventing metallic components from corroding as abovedescribed. In the arrangement illustrated a corrosion preventingelectrode 161 or 162 is electrically insulatingly extended and sealedthrough that wall portion of the flow confining tube 20 or 21 facing theinside of the gap forming surface of the main electrode 2 or 1, that is,each of the opposite surfaces of both main electrodes have anelectrically insulating holder 163 or 164 hermetically interposedtherebetween. The electrode protrudes into the flow path for the heatedliquid. The anticorrosive electrode may be formed of platinum, carbon,triiron tetroxide (Fe₃ O₄) or the like. The DC source 165 or 166 isconnected across the corrosion preventing electrode 161 or 162 and theelectrode terminal 5 or 6 thereby to supply to the electrode 161 or 162a voltage higher than the voltage across the main electrodes. To thisend each of the DC source 165 or 166 includes a negative side connectedto the associated electrode terminal 5 or 6.

Then the electrode terminals 5 and 6 are connected to a control circuitidentical to that shown in FIG. 34.

In other respects, the arrangement is identical to that shown in FIG. 24except for the provision of the auxiliary electrode 46 but the mainelectrode 1, in this case, is made of stainless steel or the like, theflow confining tube 21, the blind cover plate 23, the connecting tube36, the insulating tubes 38 and 40, the inflow tube 42 and the outflowtube 44 form an assembly prevented from corroding and generallydesignated by the reference numeral 167. Also, the similar components 2,20, 22, 35, 37, 39, 41 and 43 form another assembly prevented fromcorroding and generally designated by the reference numeral 168. Themain electrode 2 is also made of stainless steel.

The corrosion of the main electrodes and others is called electrolyticcorrosion resulting from a flow of current therethrough via a heatedelectrolytic liquid that is caused from the dissolution of materialsforming the main electrode and others into an electrolyte such as water.In the arrangement of FIG. 69, the DC sources 165 and 166 are adapted toapply to the respective corrosion preventing electrodes 161 and 162voltages higher that the voltage applied across the associated mainelectrodes. Thus the corrosion preventing electrodes 161 and 162 providethe so-called scapegoat electrodes. That is, the material or materialsforming the scapegoat electrode is or are dissolved into an electrolytesuch as water thereby to prevent the materials forming the assemblies167 and 168 from dissolving into the heated liquid resulting in nocorrosion occurring.

The DC sources 165 and 166 may be omitted by forming the corrosionpreventing electrode of a metallic material lower in corrosion potentialand more easily ionized than the material of the main electrode. Forexample, with the main electrodes 1 and 2 formed of stainless steel,magnesium, zinc, aluminum, etc. are optimum for forming the corrosionpreventing electrode.

Also the DC source may be replaced by any suitable source for supplyinga DC voltage.

FIG. 70 shows corrosion preventing electrodes provided on thearrangement shown in FIG. 39. In FIG. 70 the corrosion preventingelectrodes 161 and 162 are provided on the exposed portion of the feedwater tube 20 disposed within the main electrode 2 and on the outer wallof the main electrode 1 respectively in the same manner as abovedescribed in conjunction with FIG. 69.

Then the corrosion preventing electrodes 161 and 162 are connected toterminals d and e which are subsequently connected, for example, to theDC sources 165 and 166 (see FIG. 69) respectively. Also terminals a andb connected to the electrode terminals 5 and 6 respectively areconnected across the AC source 31 shown in FIG. 69 while a terminal Cconnected to the auxiliary electrode 46 is connected to the auxiliarysource circuit 50 also shown in FIG. 69.

FIG. 71 shows anticorrosive electrodes provided on the arrangementillustrated in FIG. 64. As shown, an anticorrosive electrode 161U, 161Vor 161W is electrically insulatingly extended and sealed through thefeed water tube 42U, 42V or 42W operatively coupled with each mainelectrode 1U, 1V or 1W with an electrically insulating holder 164U, 164Vor 164W interposed therebetween.

FIG. 72 shows a separate modification of the present invention wherein atemperature of a heated liquid is measured. In the arrangementillustrated, a temperature sensor 169 such as a thermistor iselectrically insulatingly extended and sealed through that portion of aflow confining tube 20 facing the peripheral wall of the main electrode2 with an electrically insulating holder 170 interposed therebetween.

The temperature sensor 169 may be entirely covered with a electricallyinsulating material in accordance with the particular electric fieldestablished in the vicinity thereof.

In other respects, the arrangement is substantially identical to thatshown in FIG. 24 except for the provision of the auxiliary electrode 46.

The electrode terminals 5 and 6 and the auxiliary electrode 46 areconnected to a control circuit identical to that shown in FIG. 57 exceptfor the omission of the zero volt firing circuit 90. The temperaturesensor 169 includes an output connected to the trigger circuit 99 forthe thyristor 98.

In operation the temperature sensor 169 senses the temperature of theheated liquid and feeds a measured temperature signal to the triggercircuit 99. More specifically, with the temperature sensor 169 formed ofa thermistor or a temperature measuring resistor, a resistance thereofis changed with a temperature so that a signal representative of achange in resistance is applied to the trigger circuit 99.Alternatively, with the temperature sensor 169 formed of a thermocouple,it responds to the temperature of the heated liquid to change inthermoelectromotive force thereof. This change in thermoelectromotiveforce is signalled to the trigger circuit 99.

If it is desired to control the heated liquid to a predetermined fixedtemperature then the actual temperature measured by the temperaturesensor 169 is compared with an output therefrom at a predeterminedtemperature as the reference. With the actual temperature is higher thanthe predetermined temperature, the trigger circuit 99 applies no triggersignal to the thyristor 98. Otherwise, the trigger circuit 99 deliversthe trigger signal to the thyristor 98. Then the thyristor 98 iscorrespondingly turned on and off to fire and extinguish a pilot glowdischarge thereby to effect the ON-OFF control of a glow dischargebetween the main electrodes 1 and 2.

Under these circumstances, some time goes until heat from either of themain electrodes 1 and 2 acting as the heating surface is transferred tothe heated liquid. This results in a time delay with which thetemperature of the heated liquid is controlled. Therefore, thetemperature sensor 169 has preferably a sensor end located as near tothe heating surface of the associated main electrode as possible.

With the temperature of the heated temperature controlled according to apredetermined program, the function of effecting such control may beincorporated into the trigger circuit 99 and the thyristor 98 isoperated in the ON-OFF control mode and in accordance with the outputsignal from the temperature sensor 169.

The temperature sensor 169 may be used with the control of the glowdischarge effected by a control such as a thyristor connected in seriesto the particular glow discharge heating apparatus in a circuit with anelectric source circuit for the apparatus.

The arrangement illustrated in FIG. 73 is different from that shown inFIG. 72 only in that in FIG. 73 a bidirectional triode thyristor or aTriac is provided to control the glow discharge as in the arrangement ofFIG. 48. In FIG. 73 a thyristor 172 is connected at the anode electrodeto one of the DC output terminals of the rectifier bridge 66 and at thecathode electrode to the Triac 60. The resistor 67 is connected to theTriac 60 at the gate electrode but not to one main electrode thereof.Then the thyristor 172 has the cathode and gate electrodes connectedacross a trigger circuit 173 subsequently connected to the temperaturesensor 172.

In other respects, the control circuit is substantially identical tothat shown in FIG. 48. However the dot convention is used to identifythe polarity of the instantaneous voltage across the associatedtransformer winding.

The electrically isolating transformer 65 is operative to adapt apotential difference developed across the resistor 64 to a voltagerequired for the Triac 60 to be fired.

With the switch 51 put in its closed position, the step-up transformer70 applies a high AC voltage across the electrodes 1 and 2 resulting inthe discharge breakdown occurring therebetween. This cause a potentialdifference across the current limiting resistor 64 whereby a potentialdifference appears across the resistor 67 through the transformer 65 andthe rectifier bridge 66. At that time, the trigger circuit 173 isactuated to put the thyristor 172 in its ON state to cause a triggersignal to be applied to the gate electrode of the Triac 60 to turn iton. Therefore the AC source across the source 31 is supplied across themain electrodes 1 and 2 through the now conducting thyristor 60 to causea glow discharge therebetween.

Under these circumstances, the temperature sensor 169 senses atemperature of a heated liquid involved and feeds a signal for thesensed temperature to the trigger circuit to control the glow dischargebetween the electrodes 1 and 2.

From the foregoing it is seen that in the arrangements shown in FIGS. 72and 73 the temperature of the heated liquid sensed by the temperaturesensor is fed to the auxiliary source circuit for controlling the glowdischarge caused between the main electrodes resulting in the easy,reliable temperature control of the heated liquid.

While the present invention has been illustrated and described inconjunction with various preferred embodiments thereof it is to beunderstood that numerous changes and modifications may be resorted towithout departing from the spirit and scope of the present invention.For example, the embodiments of the present invention illustrated anddescribed in conjunction with the single-phase source may readily bemodified to be driven by the three-phase source. Similarly, theembodiments illustrated and described in conjunction with thethree-phase source may readily be suited for use with polyphase sourceshaving m phases where m is greater than three (3). In the latter case,an m-phase AC voltages is applied to m main electrodes to causesuccessively glow discharges between the pairs thereof. The resultingpower capacity is equal to m times that provided by a single-phaseapparatus leading to inexpensive structures.

What we claim is:
 1. An electric shock preventing device for a glowdischarge heating apparatus for heating a heated liquid with thegeneration of heat attendant upon a glow discharge caused between a pairof discharge electrodes through which a heated liquid such as waterflows, characterized in that an electrically insulating pipe is providedon each of an inflow port and an outflow port for the heated liquid ofone of said electrodes, a metallic pipe is connected to an extremity ofeach electrically insulating pipe remote from said electrodes, and themetallic pipes are connected to ground.
 2. A glow discharge heatingapparatus as claimed in claim 1 wherein said discharge electrodes areprovided on portions thereof opposite to each other with concave andconvex areas.
 3. A glow discharge heating apparatus as claimed in claim2 wherein said concave and convex area is arranged into a plurality ofannuli, said annuli being in the form of concentric circles.
 4. A glowdischarge heating apparatus as claimed in claim 2 wherein said concaveand convex area is in the form of straight line running substantially inparallel relationship.
 5. A glow discharge heating apparatus as claimedin claim 2 wherein said concave and convex area includes a plurality ofdepressions extending into said discharge electrode.
 6. A glow dischargeheating apparatus as claimed in claim 1 wherein a tubular member isdisposed in each of said discharge electrodes to abut against aninternal wall surface of each of said discharge electrodes and saidliquid flows through said tubular member.
 7. A glow discharge heatingapparatus as claimed in claim 6 wherein said tubular member comprises aseamless copper tube brazed to the internal wall surface of theassociated discharge electrodes.
 8. A glow discharge heating apparatusas claimed in claim 1, wherein a heating space is formed within saiddischarge electrodes to cause said liquid to flow through said heatingspace while contacting directly the internal surface of each of saiddischarge electrodes thereby to be heated by said discharge electrodes,and wherein said heating space includes a normally closed exhaust portcapable of being opened to permit the liquid to be exhausted in saidheating space.
 9. A glow discharge heating apparatus as claimed in claim1, wherein a heating space is formed within said discharge electrodes tocause said liquid to flow through said heating space while contactingdirectly the internal surface of each of said discharge electrodesthereby to be heated by said discharge electrodes, and wherein acorrosion preventing electrode is disposed on each of said dischargeelectrodes to be electrically insulated from the associated dischargeelectrode and contacted by said liquid.
 10. A glow discharge heatingapparatus as claimed in claim 9 wherein said corrosion preventingelectrode has applied thereto a voltage higher than a voltage appliedacross said discharge electrodes.
 11. A glow discharge heating apparatusas claimed in claim 9 wherein said corrosion preventing electrode isformed of a metallic material less in corrosion potential and moreeasily ionized than a metallic material forming said dischargeelectrodes.
 12. A glow discharge heating apparatus, comprising:a pair ofmetallic discharge electrodes electrically biased in use to develop aglow discharge therebetween, one of said discharge electrodes operatingas a cathode and defining a fluid conduit having an inlet and an outletfor flowing a fluid therethrough and for heating the fluid when saidpair of discharge electrodes are electrically biased to develop a glowdischarge therebetween; a pair of electrically insulative fluid conduitsections each connected to a respective one of the cathode dischargeelectrode inlet and outlet for defining electrically insulativeextensions of the fluid conduit through said cathode dischargeelectrode; metallic fluid conduit sections connected to respective endportions of said electrically insulative fluid conduit sections remotefrom said cathode discharge electrode for defining electricallyconductive extension of the fluid conduit through said cathode electrodeand said electrically insulative fluid conduit sections; means forgrounding said metallic fluid conduit sections to prevent electricalshock upon contact with said metallic fluid conduit sections; andwherein said pair of electrically insulative fluid conduit sections aresufficiently long to reduce a leakage current from said dischargeelectrodes through said electrically insulative fluid conduit sectionsto ground to a value less than a certain predetermined current value.13. A glow discharge heating apparatus according to claim 9, whereinsaid pair of metallic discharge electrodes each comprise a fluid conduitto permit fluid heating by both of said pair of metallic dischargeelectrodes when said pair of metallic discharge electrodes are biased byan A.C. voltage sufficient to develop a glow discharge.
 14. A glowdischarge heating apparatus according to claim 13, wherein respectiveportions of said pair of discharge electrodes face each other and definea gap therebetween wherein a glow discharge is formed, in use; andwherein the respective facing portions of said pair of dischargeelectrodes are corrugated to increase the surface areas of therespective facing portions of said pair of discharge electrodes.
 15. Aglow discharge heating apparatus according to claim 12, 13 or 14,further comprising: a respective corrosion preventing electrode disposedextending into each discharge electrode defining a fluid conduit forcontacting fluid therein; and insulating means electrically insulatingeach corrosion preventing electrode from the respective dischargeelectrode into which it extends.